Windows users: Right-click over the image and choose "Set as Background."
Mac users: Control-click on the link below and select "Download Image to Disk" (IE) or "Save this Image as…" (Netscape). Save the file to a location that you will remember. Then go to the Appearance Control Panel, and choose the "desktop" tab. Click on "Place Picture…" and choose the picture you just saved. Click on "Set Desktop."