Select the size closest to the size of your computer desktop resolution.

Once the image has loaded, then:

Windows Users: Right-click it and choose Set as Background or Set as Wallpaper.

Mac OS X Users: Drag it onto your desktop, go to System Preferences, go to the Desktop icons, click the Collection drop-down and Choose Folder to find the new wallpaper on your computer.

Mac OS 9 Users: Drag it onto your desktop, go to Control Panel and choose Appearance, click Set Desktop and choose the new wallpaper that you just downloaded.

800 x 600	800 x 600
1024 x 768	1024 x 768
800 x 600	800 x 600
1024 x 768	1024 x 768