Аспект изображения для растяжения

<phone:PhoneApplicationPage
    x:Class="firstApp.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
    xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:uc="clr-namespace:firstApp"
    mc:Ignorable="d"
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeNormal}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    SupportedOrientations="Portrait" Orientation="Portrait"
    shell:SystemTray.IsVisible="True">
 
    <!--LayoutRoot представляет корневую сетку, где размещается все содержимое страницы-->
    <Grid x:Name="LayoutRoot" Background="Transparent">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
 
       
        <StackPanel x:Name="ContentPanel" Grid.Row="1" >         
            <Image x:Name ="image"  CacheMode="BitmapCache"  Height="544"  DoubleTap="ChoosePhoto_Click"/>
            <Button x:Name ="runButton" Content="Glitch!" HorizontalAlignment="Stretch"  Click="runButton_Click"/>
            <StackPanel x:Name="ContentPanelHorizontal"  Orientation="Horizontal" Grid.Row="1" >
                <CheckBox x:Name="chScan"  Content="Scan Lines" IsChecked="True" Width="180" />
                <CheckBox x:Name="chOldTv" Content="Old TV" Width="150" />
                <CheckBox x:Name="chDistort" Content="Cut" IsChecked="True" Width="118"  />
            </StackPanel>
        </StackPanel>
    </Grid>

<UserControl x:Class="firstApp.UCImage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Name="uc" RenderTransformOrigin="0.5,0.5">  
  <UserControl.RenderTransform>
    <CompositeTransform/>
  </UserControl.RenderTransform>
 
  <Grid>
    <ViewportControl x:Name="viewport"
                     ManipulationStarted="OnManipulationStarted" ManipulationDelta="OnManipulationDelta"
                     ManipulationCompleted="OnManipulationCompleted" ViewportChanged="viewport_ViewportChanged" Background="#FFE6C7C7">
      <Canvas x:Name="canvas" >
                <Image x:Name="image"  
                       CacheMode="BitmapCache" RenderTransformOrigin="0,0"
               Source="{Binding ImageSource,ElementName=uc}" >
          <Image.RenderTransform>
            <CompositeTransform x:Name="compositeRenderTransform"/>
          </Image.RenderTransform>
        </Image>
      </Canvas>
    </ViewportControl>
  </Grid>
</UserControl>