I have written a simple example that uses the SlideShow extender and demonstrates some of its cool features.
In the example, I attach a handler to the 'slideChanging' event in the pageLoad function. The handler updates the previous image using the SlideShowEventArgs parameter that is passed to it. SlideShowEventArgs exposes the previous and next slide objects out of which I use the previous slide to update the previous image. Check out the attached zip. It just a proof of concept, not super-refined and could do with a lot of help in the visual department.
Are there other simple customizations of Toolkit controls that you would like to see? Let us know.