PC: Click on appropriate size, right
click image, choose 'Set As Wallpaper/Background'
from Menu. OR right click image, choose
'Same Image As' from menu and
save in 'My Pictures'. You
will be able to save multiple
wallpapers and switch between
them by changing the wallpaper
selection of your desktop properites.
Mac OS 9: Save image, open Appearance
control panel. Select Desktop folder tab, Place
Picture.
Mac OS X: Open System Preferences, click
on Desktop settings. Drag image from desktop
to preview box in Desktop Preferences window.