Create Background Slideshow
For the desktop background, GNOME is able to use a slideshow in which images automatically change over time. (The "Cosmos" set is the standard example.) Create Background Slideshow (CreBS) can be used to create new sequences for the desktop wallpaper, by selecting images and specifying their timings. CreBS then creates the wallpaper XML files that GNOME uses in order to change the backgrounds automatically.
As with static wallpapers, the slideshows will appear in the standard "Appearance Preferences" window, under the "Background" tab.