My background is currently from Thief 3: Deadly Shadows. http://www.thief3.com/gallery/wallpaper_01_1024x768.jpg
Warning, it is big.
I used to have a map of Middle Earth for my background, but I got tired of it and changed it.
I am not using a screensaver however, my computer just goes into low power mode or whatever it's called
A Sluggy Freelance strip from a week or two ago ("Come on, Sam's not a crier. Sam's da man!") is my desktop wallpaper, and my screensaver is the 3d Flower Box that comes with Windows.