Every robot needs some good wallpaper. I tend to change my desktop wallpaper at least once a week, so I have a variety of sources from which I get good wallpaper. I also require a wide variety of wallpaper because I use many different desktop aspect ratios including standard, widescreen, and dual-monitor. These are the sites that I have found to be best for high-quality wallpapers:
Standard or Widescreen:
Yahoo seems to have the best image search because they allow you to search for images that have the aspect ratio of a desktop background (click the wallpaper button in the top right corner).