@Krakerman The prior code @faeran gave you for you game specific backgrounds will work as well. Just make a little edit to the source and change the path to the correct folder structure and change the file type to .gif. Something like below. The x:Name can be anything you want to call it. Just make sure in the "ElementName" you reference the same name there. I use something similar to this for gifs.
<TextBlock x:Name="GameGifs" >
<TextBlock.Text>
<MultiBinding StringFormat="{}pack://siteoforigin:,,,/StartupThemes/LBFade/Images/Gifs/{0}/{1}.gif">
<Binding Path="SelectedGame.Platform" />
<Binding Path="SelectedGame.Title" />
</MultiBinding>
</TextBlock.Text>
</TextBlock>
<Image gif:ImageBehavior.AnimatedSource="{Binding Text, ElementName=GameGifs}" Stretch="UniformToFill" RenderOptions.BitmapScalingMode="HighQuality"
Height="{Binding ElementName=Canvas, Path=ActualHeight}" Width="{Binding ElementName=Canvas, Path=ActualWidth}" />