|1) Choose the image in the appropriate size
for your monitor
2) Once the image has fully downloaded, right
click (or command key) and select "Save
image to desktop"
3) Once the image saves to your desktop, Choose
System Preferences from the Apple Menu
4) Click on "Desktop & Screen Savers".
Click on the DESKTOP tab.
5) Click on "Choose Folder" from
the left menu to locate your image. (It saves
to your destop by default)
6) Click on the image to set it as your background.