Select your favorite picture by clicking on the desired size (a popup will appear), save the image to your hard drive (instructions below), and then make it your desktop image.
Select image size:
INSTRUCTIONS
= 1024x768 (100K)
= 1280x1024(140K)
PC users: right click on the image and select from the menu "Save as desktop item..."
Mac users (OSX): hold click down on image and save to disk. Go to the System Preferences and select Desktop Images. Follow instructions.