In a way, wallpapers are very much like electronic Vision Boards. What I've done is to use a software to help me create it and reextract it as a screensaver and a wallpaper.
As for photos, another resource is
stock.xchng. You can get some nice quality royalty free photos for free. I've used some of them in my Vision Board experiment and I must admit the quality for most of the photos are quite good.