From Brian Bergstrand:
Change Desktop opens your Desktop to all new possibilities. Unlike the System desktop changer, you can mix and match any iPhoto album, Aperture album, folder or single image to display on your Desktop.
Changes your Desktop picture at a specified interval on up to eight monitors. It installs a system wide hot key so you change your Desktop picture from any application. ChangeDesktop can choose a picture at random, or choose each picture in sequence. If pictures are chosen at random, ChangeDesktop can be configured to use all pictures before repeating any picture. Coalesces pictures from multiple folders so you don't have to move all of your pictures to one folder. Sub-folders are also searched.