Wile there’re many desktop themes available for Windows 7, including Paul Thurrott’s collection and various OSX inspired theme packs. But do you know, you can specify a RSS feed as the source of slideshow images, as part of the new theme file specifications in Windows 7.
First of all, this feature utilizes the Windows RSS Platform which automatically refreshes and download feed enclosures in the background. Because this is a background process, it will take considerable time to download the high-resolution photos within the feed. But once the photos are downloaded, the theme does not automatically refresh to queue the new photos in the slideshow. A logout/login should be sufficient, but more simply you could open the theme control panel and toggle between two themes to force a manual refresh.