Desktop Slideshow, at least in Windows 7 build 6801, supports changing of the desktop wallpaper at intervals ranging from 10 seconds to 1 day. For a bit of variation, you can tick the Shuffle checkbox to have Windows 7 randomly pick a wallpaper for you. And to keep from sapping the life out of your laptop, you can choose to pause the slideshow when you’re on limited reserve. Think of this as Dreamscene, but without the ensuing nightmares.
For those of us that don’t have terabytes of imagery to keep your desktop fresh, you have a new source in the Picture location dropdown labeled Feeds.
From this interface, you can choose an RSS feed and automatically download fresh images, like NASA’s Large Image of the Day, to your desktop. At time of writing, however, this feature was not completely functional. I suspect this feature will either work at the conclusion of my OS-wide protection witch hunt or not work at all, being a pre-beta build and all.