Click on the link corresponding to your screen resolution. The image will open in a new window.

PC: Right-click on the image and choose "Set As Background".

MAC OS X: Control-click and select "Download image to Disk". Save image to desktop. Go to Apple menu > System Preferences > Personal section and select "Desktop". On the "Collection" pulldown menu, go to "Choose Folder" and select the wallpaper.

MAC OS9: Control-click and select "Save this Image as" to your Mac. Go to Apple menu > Control Panel and select "Appearance". Select "Place picture" and select the wallpaper. Click "Set Desktop".