Instructions
PC
Right click on the image and click "Set as Background"
Mac (OS 9.x)
Drag the image onto your desktop, then click the "Desktop" tab in the Appearance control panel. Drag the picture onto the sample screen, and click "Set Desktop."
Mac (OS 10.x)
Drag the image onto your desktop, then open System Preferences and click "Desktop." Drag the image from your desktop to the "Current Desktop Picture" sample screen.