PC / Windows users: Click on the resolution that best fits your screen, then right click on the image and select 'set as wallpaper' or 'set as background'.
MAC OSX users: Click the resolution suitable for your screen, then drag the image off onto your desktop. Go to System Preferences. Go to the Desktop icon and open it. Choose the Collection drop down and Choose Folder to find your new Wallpaper on your computer.
MAC OS9 users: Click the resolution suitable for your screen, then drag the image off onto your desktop. Go to your Control Panel and choose Appearance. Click 'set desktop' and choose your newly downloaded image on your desktop. Close the window to return here.