While UltraMon (or DisplayFusion) is useful if you only have a few different images you'll manually be placing on your desktop from time to time, I recommend Wallpaper Master Pro (http://jamesgart.com/wallpaperchanger/index.html) for people who have lots of dual/spanning images across many directories. I've used everything out there on my five monitor desktop and the only wallpaper changer I purchased was this one. Enjoy.
