C#-Code

using System;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.IO;

namespace aufgabe_89_wpf     {

    public partial class MainWindow : Window    {

        public MainWindow()        {
            InitializeComponent(); this.UpdateLabels();
        }

        #region some helper methods
        private static Microsoft.Win32.OpenFileDialog opendialog(string filter)        {
            Microsoft.Win32.OpenFileDialog dlg = new Microsoft.Win32.OpenFileDialog();
            dlg.DefaultExt = ".png";
            dlg.Filter = filter + "all|*.*";
            return dlg;
        }

        private void UpdateLabels()        {
            this.lb_loaded_picture.Content  = 
                System.IO.Path.GetFileName(this.loaded_picture_file);
            this.lb_loaded_and_discovered_picture.Content   = 
                System.IO.Path.GetFileName(this.loaded_and_discovered_picture_file);
        }
        #endregion

        #region some global variables
        private static string NL = Environment.NewLine;

        private string loaded_picture_file                  = string.Empty;
        private string loaded_text_file                     = string.Empty;
        private string loaded_and_discovered_picture_file   = string.Empty;

        private bool bo_show_discovery_after_creation       = false;
        #endregion

        #region event handler
        private void bt_reset_Click(object sender, RoutedEventArgs e)
        {
            this.image_org.Source = null;
            this.image_hid.Source = null;
            this.tb_hide_info.Clear();
            this.tb_discovered_info.Clear();
            this.tb_helper_links.Clear();
            this.tb_helper_rechts.Clear();
            this.loaded_picture_file = string.Empty;
            this.loaded_text_file = string.Empty;
            this.loaded_and_discovered_picture_file = string.Empty;
            this.UpdateLabels();
        }

        private void bt_load_picture_Click(object sender, RoutedEventArgs e)
        {
            Microsoft.Win32.OpenFileDialog dlg = opendialog("special graphics|*.bmp;*.jpg;*.jpeg;*.png|");
            if(dlg.ShowDialog() == true)            {
                string filename = dlg.FileName;
                this.loaded_picture_file = filename;
                this.UpdateLabels();

                BitmapImage bi = new BitmapImage(new Uri(filename));                
                this.image_org.Source = bi;                
            }
        }

        private void bt_hide_text_Click(object sender, RoutedEventArgs e)
        {
            if (this.loaded_picture_file == string.Empty)
                return;

            try            {
                BitmapImage bi = new BitmapImage(new Uri(this.loaded_picture_file));

                BitmapSource bitmapSource = new FormatConvertedBitmap(bi, PixelFormats.Default, null, 0);
                WriteableBitmap wbi = new WriteableBitmap(bitmapSource);

                int h = wbi.PixelHeight;
                int w = wbi.PixelWidth;
                int widthInByte = 4 * w;

                int[] pixelData = new int[w * h];
                wbi.CopyPixels(pixelData, widthInByte, 0);

                byte[] target = new byte[4 * w * h];
                for (var i = 0; i < pixelData.Length; i++)                {
                    Byte[] fourbytes = BitConverter.GetBytes(pixelData[i]);
                    fourbytes.CopyTo(target, 4 * i);
                }

                char[] chars = this.tb_hide_info.Text.ToCharArray();
                Encoding enc = Encoding.UTF8;// BigEndianUnicode;
                byte[] source = enc.GetBytes(chars);
                byte[] new_target = target.Hide(source);

                wbi.WritePixels(new Int32Rect(0, 0, w, h), new_target, widthInByte, 0);
                this.image_hid.Source = wbi;

                this.tb_helper_links.Text = new_target.ToMyString();

                byte[] hidden_txt = new_target.Discover();

                char[] hidden_chars = enc.GetChars(hidden_txt);
                string s = new string(hidden_chars);

                string hidden_file = System.IO.Path.GetDirectoryName(this.loaded_picture_file) + "\\hidden.bmp";
                this.loaded_and_discovered_picture_file = hidden_file;
                UpdateLabels();
                BitmapSource myBitmapImage = this.image_hid.Source as BitmapSource;

                if (File.Exists(hidden_file))
                    File.Delete(hidden_file);
                myBitmapImage.Save(hidden_file);

                if (bo_show_discovery_after_creation)
                    this.tb_discovered_info.Text = s;
            }
            catch (Exception ee)            {
                this.tb_discovered_info.Text = "not possible to save the file" + NL + NL + ee.Message;
            }
        }

        private void bt_load_text_Click(object sender, RoutedEventArgs e)
        {
            Microsoft.Win32.OpenFileDialog dlg = opendialog("txt|*.txt|");
            if (dlg.ShowDialog() == true)
                this.tb_hide_info.Text = System.IO.File.ReadAllText(dlg.FileName);
        }

        private void bt_load_and_discover_picture_Click(object sender, RoutedEventArgs e)
        {
            Microsoft.Win32.OpenFileDialog dlg = opendialog("graphics|*.bmp|");
            if (dlg.ShowDialog() == true)            {
                string filename = dlg.FileName;
                this.loaded_and_discovered_picture_file = filename;
                this.UpdateLabels();

                BitmapImage bi = new BitmapImage(new Uri(filename));
                this.image_hid.Source = bi;

                BitmapSource bitmapSource = new FormatConvertedBitmap(bi, PixelFormats.Default, null, 0);
                WriteableBitmap wbi = new WriteableBitmap(bitmapSource);

                int h = wbi.PixelHeight;
                int w = wbi.PixelWidth;
                int widthInByte = 4 * w;

                int[] pixelData = new int[w * h];
                wbi.CopyPixels(pixelData, widthInByte, 0);

                byte[] target = new byte[4 * w * h];
                for (var i = 0; i < pixelData.Length; i++)                {
                    Byte[] fourbytes = BitConverter.GetBytes(pixelData[i]);
                    fourbytes.CopyTo(target, 4 * i);
                }
                this.tb_helper_rechts.Text = target.ToMyString();                
                try                {
                    byte[] hidden_txt = target.Discover();

                    Encoding enc = Encoding.UTF8;
                    char[] hidden_chars = enc.GetChars(hidden_txt);
                    string s = new string(hidden_chars);

                    this.tb_discovered_info.Text = s;
                    this.UpdateLabels();
                }
                catch (Exception ee)                {
                    this.tb_discovered_info.Text = "no hidden infos discovered" + NL + NL + ee.Message;
                }
            }
        }
        #endregion
    }

    public static class Hide_and_Discover    {

        public static Byte[] Hide(this Byte[] hidden_place, Byte[] source)        {
            if (hidden_place.Length < 4 * (4 + source.Length))            {  //die ersten 16 Byte enthalten die arraylaenge von source
                MessageBox.Show("Inputerror Hide: " + hidden_place.Length.ToString() + " " + source.Length.ToString());
                return hidden_place;
            }

            Byte[] first4_source = BitConverter.GetBytes(source.Length);
            Byte[][] targets = hidden_place.Splitt(4 * first4_source.Length);

            targets[0] = BytesIntoBytes(targets[0], first4_source);
            targets[1] = BytesIntoBytes(targets[1], source);
            return targets.Join();
        }

        public static Byte[] Discover(this Byte[] source)        {

            Byte[] first16 = source.GetSubArray(anzahl: 16);
            Byte[][] bint = new Byte[4][];
            for (var i = 0; i < 4; i++)            {
                bint[i] = new Byte[4];
                for (var j = 0; j < 4; j++)
                    bint[i][j] = first16[4 * i + j];
            }
            Byte[] forint = new Byte[4];
            for (var i = 0; i < 4; i++)
                forint[i] = bint[i].From4();

            forint.Reverse();
            int size = BitConverter.ToInt32(forint, 0);

            Byte[] result = new Byte[size];
            int zeiger = 16;
            for (var i = 0; i < size; i++)            {
                Byte[] next4 = source.GetSubArray(ab: zeiger, anzahl: 4);
                result[i] = next4.From4();
                zeiger += 4;
            }
            return result;
        }

        private static Byte[] BytesIntoBytes(this Byte[] hidden_place, Byte[] source)        {

            Byte[] result = new Byte[hidden_place.Length];
            hidden_place.CopyTo(result, 0);

            Byte eliminate_last_two = 0xFC;
            Byte[] filter = new Byte[] { 0x03, 0x0C, 0x30, 0xC0 };

            int counter = 0;
            for (var i = 0; i < source.Length; i++)            {
                Byte b = source[i];
                for (Byte j = 0; j < filter.Length; j++)                {
                    Byte bs = (Byte)(((Byte)(b & filter[j])) >> (2 * j));
                    result[counter] = (Byte)((Byte)(result[counter] & eliminate_last_two) ^ bs);
                    counter++;
                }
            }
            return result;
        }

        private static Byte[][] Splitt(this Byte[] b, int ab)        {
            Byte[][] result = new Byte[2][];
            result[0] = new Byte[ab];
            for (var i = 0; i < result[0].Length; i++)
                result[0][i] = b[i];
            result[1] = new Byte[b.Length - ab];
            for (var i = 0; i < result[1].Length; i++)
                result[1][i] = b[i + ab];
            return result;
        }

        private static Byte[] Join(this Byte[][] b)        {
            Byte[] result = new Byte[b[0].Length + b[1].Length];
            b[0].CopyTo(result, 0);
            b[1].CopyTo(result, b[0].Length);
            return result;
        }

        private static Byte From4(this Byte[] b)        {
            Byte maske = 0x03, result = 0x00;
            for (var i = 0; i < b.Length; i++)            {
                Byte value = (Byte)((b[i] & maske) << 2 * i);
                result = (Byte)(result ^ value);
            }
            return result;
        }

        private static Byte[] GetSubArray(this Byte[] b, int ab = 0, int anzahl = 1)        {
            Byte[] result = new Byte[anzahl];
            for (var i = 0; i < anzahl; i++)
                result[i] = b[i + ab];
            return result;
        }
    }

    public static class MyExtensions    {

        public static string ToMyString(this byte[] b)        {
            StringBuilder sb = new StringBuilder();
            sb.AppendLine(b.Length.ToString("n0"));
            for (var i = 0; i < b.Length; i++)            {
                if (i % 16 == 0)
                    sb.AppendLine();
                sb.Append(String.Format("{0:X2} ", b[i]));
            }
            return sb.ToString();
        }

        public static void Save(this BitmapSource image, string filePath)        {
            BitmapEncoder encoder = new BmpBitmapEncoder();// PngBitmapEncoder();
            encoder.Frames.Add(BitmapFrame.Create(image));
            using (var fileStream = new System.IO.FileStream(filePath, System.IO.FileMode.Create))            {
                encoder.Save(fileStream);
            }
        }
    }
}

C#-Code

<Window x:Class="aufgabe_89_wpf.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:aufgabe_89_wpf"
        mc:Ignorable="d"
        Title="hide and discover demo" Height="600" Width="970" FontFamily="Courier New" Background="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}" WindowState="Maximized">
    <Grid>
        <TabControl VerticalContentAlignment="Stretch" HorizontalContentAlignment="Stretch" TabStripPlacement="Bottom">
            <TabItem Header=" main ">
                <Grid x:Name="grid_tab_0" Margin="0,3,0,0" Background="{DynamicResource {x:Static SystemColors.GradientInactiveCaptionBrushKey}}">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="1*"/>
                        <ColumnDefinition Width="1*"/>
                    </Grid.ColumnDefinitions>
                    <Grid.RowDefinitions>
                        <RowDefinition Height="45"/>
                        <RowDefinition Height="1*"/>
                        <RowDefinition Height="1*"/>
                        <RowDefinition Height="45"/>
                    </Grid.RowDefinitions>
                    <GridSplitter Grid.Column="1" HorizontalAlignment="Left" Margin="0,10" Grid.Row="1" Grid.RowSpan="3" Width="5"/>
                    <GridSplitter Grid.Row="1" ResizeDirection="Rows" HorizontalAlignment="Stretch" Height="5" VerticalAlignment="Bottom" Grid.ColumnSpan="2"/>
                    <Image x:Name="image_org" Margin="10" Grid.Row="1"/>
                    <Image x:Name="image_hid" Margin="10" Grid.Row="1" Grid.Column="1"/>
                    <StackPanel Margin="5,15,5,5" Orientation="Horizontal">
                        <Button x:Name="bt_reset" Content=" reset " HorizontalAlignment="Center" VerticalAlignment="Center" Margin="10,0,25,0" Click="bt_reset_Click"/>

                        <Button x:Name="bt_load_picture" Content=" load picture " HorizontalAlignment="Center" VerticalAlignment="Center" Margin="10,0,0,0" Click="bt_load_picture_Click"/>
                        <Button x:Name="bt_load_text" Content=" load text " HorizontalAlignment="Center" VerticalAlignment="Center" Margin="10,0,0,0" Click="bt_load_text_Click"/>
                        <Button x:Name="bt_hide_text" Content=" hide text from textbox " HorizontalAlignment="Center" VerticalAlignment="Center" Margin="50,0,0,0" Click="bt_hide_text_Click"/>
                    </StackPanel>
                    <StackPanel Margin="5,15,5,5" Orientation="Horizontal" Grid.Column="1">
                        <Button x:Name="bt_load_and_discover_picture" Content=" load picture and discover into textbox " HorizontalAlignment="Center" VerticalAlignment="Center" Margin="10,0,0,0" Click="bt_load_and_discover_picture_Click" />
                    </StackPanel>
                    <GroupBox Header=" hide information " Margin="5" Grid.Row="2" Background="{DynamicResource {x:Static SystemColors.ActiveCaptionBrushKey}}" BorderThickness="1,0,1,1" Foreground="#FFDE2020" Grid.RowSpan="1" BorderBrush="#FF171819">
                        <TextBox x:Name="tb_hide_info" Margin="5" VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto" AcceptsReturn="True" AcceptsTab="True"/>
                    </GroupBox>
                    <GroupBox Header=" discovered information " Margin="5" Grid.Column="1" Grid.Row="2" Background="{DynamicResource {x:Static SystemColors.ActiveCaptionBrushKey}}" BorderThickness="1,0,1,1" Foreground="#FFDE2020" Grid.RowSpan="1">
                        <TextBox x:Name="tb_discovered_info" Margin="5" VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto" AcceptsReturn="True" AcceptsTab="True"/>
                    </GroupBox>
                    <Label x:Name="lb_loaded_picture" Content="Label" Margin="10,0,0,0" Grid.Row="3" VerticalAlignment="Center"/>
                    <Label x:Name="lb_loaded_and_discovered_picture" Content="Label" Margin="10,0,0,0" Grid.Row="3" Grid.Column="1" VerticalAlignment="Center"/>
                </Grid>
            </TabItem>
            <TabItem Header=" extra text ">
                <Grid x:Name="grid_tab_1">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="1*"/>
                        <ColumnDefinition Width="1*"/>
                    </Grid.ColumnDefinitions>
                    <TextBox x:Name="tb_helper_links" Margin="5" VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto" AcceptsReturn="True" AcceptsTab="True" />
                    <TextBox x:Name="tb_helper_rechts" Margin="5" VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto" AcceptsReturn="True" AcceptsTab="True" Grid.Column="1"/>
                </Grid>
            </TabItem>
        </TabControl>
    </Grid>
</Window>