Open up the registry and browse to:

HKEY_USERS.DefaultControl PanelDesktop

edit the Wallpaper key to point to where ever you have your wallpaper stored (eg. c:windowswallpaper.bmp). The file needs to be a .bmp to show up but that is all you should need to do.
After a restart the background will show up behind the login window assuming you are not using the welcome screen.
If a user logs in and then logs out their wallpaper will show up instead. But this is great to keep things looking the same when computers are restarted often.