If you browse the Silverlight Community site here, you will find plenty of Slideshow type applications. If you want to extend them, you can often get the source code, but many of them are written in previous versions of Silverlight or possibly reference older .net assemblies.
Since I particularly like the one by Michiel Post from the Netherlands (you can see it on his blog here) I decided to upgrade it to Silverlight 4, which you can download from the file attached to this post. I was going to customise it, hence why the Solution name has changed from Photo Browser to Slideshow. I stopped short of customising it when I found the Vertigo slideshow solution, which is very easy to customise. It is provided on Codeplex by Vertigo here and you can see I've used it (customised of course) here.
Slideshow.zip (543.89 kb)