I had been using Microsoft's Wallpaper Powertoy to rotate my desktop backgrounds; I now use Winwall (http://net-session.com/winwall/). The biggest limitation of Microsoft's Powertoy is that it only recognises GIF (boo!), JPEG and BMP images - i.e. not PNGs. Winwall, however, recognises all four formats (and probably more), and its automatic resizing looks much nicer. The program is closed source, but freeware.