• Click on the image size desired
• Right-click the image
• Select "Set as Wallpaper"

Mac Users

• Hold the option key and select the image size
• Return to the desktop, hold the control key and click on an open space on the desktop
• Select "Change Desktop Background"
• Click the "Place Picture" button
• Browse for the image (usually on the hard drive) and click on it
• Click the "Set Desktop" button