For example my Startups and Pause theme's as well as a custom marquee xaml I use a custom folder stored in the main \Launchbox\Images folder so I do not have to have multiple copies of those images in each theme. I just point the theme xaml or marquee xaml to a custom folder in the Launchbox Image folders. Below is the code used for marquee images similar to what I do for game controls:
<TextBlock x:Name="MarqueeStatic" Visibility="Collapsed">
<TextBlock.Text>
<MultiBinding StringFormat="{}pack://siteoforigin:,,,/Images/Marquees/Platforms/{0}.png">
<Binding Path="SelectedGame.Platform" />
</MultiBinding>
</TextBlock.Text>
</TextBlock>