MainWindow.xaml

<Window x:Class="ImageMerger.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:ImageMerger"
        mc:Ignorable="d"
        Title="Image Merger" Height="660" Width="600" WindowStartupLocation="CenterScreen">

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="40"/>
            <RowDefinition/>
        </Grid.RowDefinitions>

        <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" Margin="0,5,0,5">
            <Button Click="MergeImages" Content="Merge Images" Name="Merge_Images" Margin="0,0,5,0" Padding="5,1,5,1" ToolTip="Merge 'source images' with 'merge images' into 'target images'">
                <Button.Triggers>
                    <EventTrigger RoutedEvent="Button.Loaded">
                        <BeginStoryboard>
                            <Storyboard>
                                <ColorAnimationUsingKeyFrames Storyboard.TargetName="Merge_Images" Storyboard.TargetProperty="(Background).(SolidColorBrush.Color)" Duration="0:0:1" RepeatBehavior="4x">
                                    <ColorAnimationUsingKeyFrames.KeyFrames>
                                        <LinearColorKeyFrame Value="LightPink" KeyTime="0:0:0.5" />
                                        <LinearColorKeyFrame Value="LightGray" KeyTime="0:0:1" />
                                    </ColorAnimationUsingKeyFrames.KeyFrames>
                                </ColorAnimationUsingKeyFrames>
                            </Storyboard>
                        </BeginStoryboard>
                    </EventTrigger>
                </Button.Triggers>
            </Button>
            <Button x:Name="ShowImagesButton" Click="ShowImages" Content="Show Merged Images" IsEnabled="False" Margin="0,0,5,0" Padding="5,1,5,1" ToolTip="Watch the 'target images' stacked on top of each other"/>
            <Button x:Name="SaveImagesButton" Click="SaveImages" Content="Save Merged Images" IsEnabled="False" Margin="0,0,5,0" Padding="5,1,5,1" ToolTip="Save the 'target images' to a PDF file"/>
            <Button Click="ResetImages" Content="Reset Images" Padding="5,1,5,1" ToolTip="Reset all displayed images to their default values"/>
        </StackPanel>

        <Viewbox Grid.Row="1">
            <Grid Grid.IsSharedSizeScope="True" ShowGridLines="True">
                <Grid.RowDefinitions>
                    <RowDefinition SharedSizeGroup="A"/>
                    <RowDefinition SharedSizeGroup="A"/>
                    <RowDefinition SharedSizeGroup="A"/>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="40"/>
                    <ColumnDefinition SharedSizeGroup="B"/>
                    <ColumnDefinition SharedSizeGroup="B"/>
                    <ColumnDefinition SharedSizeGroup="B"/>
                </Grid.ColumnDefinitions>

                <Viewbox Grid.Row="0" Grid.Column="0">
                    <Label Content="Source images" HorizontalAlignment="Center" VerticalAlignment="Center">
                        <Label.LayoutTransform>
                            <RotateTransform Angle="-90" />
                        </Label.LayoutTransform>
                    </Label>
                </Viewbox>
                <Viewbox Grid.Row="1" Grid.Column="0">
                    <Label Content="Merge images" HorizontalAlignment="Center" VerticalAlignment="Center">
                        <Label.LayoutTransform>
                            <RotateTransform Angle="-90" />
                        </Label.LayoutTransform>
                    </Label>
                </Viewbox>
                <Viewbox Grid.Row="2" Grid.Column="0">
                    <Label Content="Target images" HorizontalAlignment="Center" VerticalAlignment="Center">
                        <Label.LayoutTransform>
                            <RotateTransform Angle="-90" />
                        </Label.LayoutTransform>
                    </Label>
                </Viewbox>
                <Image x:Name="Image00" Grid.Row="0" Grid.Column="1" Width="200" Source="pack://application:,,,/Images/Image00.jpg" MouseLeftButtonUp="Image_Click"/>
                <Image x:Name="Image01" Grid.Row="0" Grid.Column="2" Width="200" Source="pack://application:,,,/Images/Image01.jpg" MouseLeftButtonUp="Image_Click"/>
                <Image x:Name="Image02" Grid.Row="0" Grid.Column="3" Width="200" Source="pack://application:,,,/Images/Image02.jpg" MouseLeftButtonUp="Image_Click"/>
                <Image x:Name="Image03" Grid.Row="1" Grid.Column="1" Width="200" Source="pack://application:,,,/Images/Image03.jpg" MouseLeftButtonUp="Image_Click"/>
                <Image x:Name="Image04" Grid.Row="1" Grid.Column="2" Width="200" Source="pack://application:,,,/Images/Image04.jpg" MouseLeftButtonUp="Image_Click"/>
                <Image x:Name="Image05" Grid.Row="1" Grid.Column="3" Width="200" Source="pack://application:,,,/Images/Image05.jpg"/>
                <Image x:Name="Image06" Grid.Row="2" Grid.Column="1" Width="200" Source="pack://application:,,,/Images/Image06.jpg"/>
                <Image x:Name="Image07" Grid.Row="2" Grid.Column="2" Width="200" Source="pack://application:,,,/Images/Image07.jpg"/>
                <Image x:Name="Image08" Grid.Row="2" Grid.Column="3" Width="200" Source="pack://application:,,,/Images/Image08.jpg"/>
            </Grid>
        </Viewbox>
    </Grid>
</Window>