Have you seen the new spectacular image from the Hubble Space Telescope? Here it is with links also to all the different forms and sizes of the image to download:
Also check out the 25 top Hubble images here:
I downloaded most of the wallpapers from all of those for my desktop resolution, put them in a folder, selected the folder in System Preferences > Desktop & Screensaver, and have them changing every hour. So cool!