MainPage.xaml

<Page
    x:Class="WriteableBitmapExBlitAlphaRepro.WinRT.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:WriteableBitmapExBlitAlphaRepro.WinRT"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
        <StackPanel Orientation="Horizontal">
            <Grid>
                <Image x:Name="ImgOrg" VerticalAlignment="Top" HorizontalAlignment="Left" Width="512" Height="512" />
                <Image x:Name="ImgOrgOverlay" VerticalAlignment="Top" HorizontalAlignment="Left" Width="150" Height="150" />
            </Grid>
            <Image x:Name="ImgMod" VerticalAlignment="Top" HorizontalAlignment="Left" Width="512" Height="512" />

        </StackPanel>
    </Grid>
</Page>