I think that a search for elements of nature on flickr is probably the most fruitful ways to find great, unique, desktop wallpaper.  Certain elements in nature are better than others, however.  I think sunlit water droplets on leaves are curiously perfect for desktop backgrounds.  Search for “leaf droplets”

Shimmering sunlit droplets with fisheye world reflection, held by surface tension to the epidermis of a leaf.  Or a series of droplets perfectly distributed by the leaf’s vein structure.  These typically macro mode photos will also have a perfectly blurred background, further enhanced if taken with SLR cameras, so they won’t obscure icons.  You really have to be a bad photographer to mess up this shot.

What other nature searches produce brilliant wallpaper?