I recently found I could use the background from Time Machine (Mac OS X 10.5) as a wallpaper… overall it looks pretty good (even on pcs). I found it when i was browsing the resources for Finder.app… here is the full path.
Here is the image in use as wallpaper:
Desktop screenshot of my MacBook… 1280×800… Mac OS X 10.5
Desktop screenshot of my Main PC… 1440×900… Windows XP SP
Desktop screenshot of my Media PC… 1280×1024… Windows XP SP3