Instructions:

Windows Users: Click on the size that matches your screen resolution most closely. Right-Click on that image and choose "Set as Background".

Macintosh OS 9.x Users: Click on the size that closely matches your screen resolution. Right-Click on the image and save it to your hard drive. Go into the Apple Menu and choose Control Panels>Appearance. Navigate to the Backgrounds tab. Drag and drop the saved image into the background drop area.

Mac OS X Users: Download the image as with OS 9. Go to System Preferences>Desktop and follow the instruction for OS 9 Users.