Have you noticed what's new with Google? Go ahead, open http://www.google.com then take a good look. Have you noticed it yet? Take a look at the bottom left of the screen... OK! OK! here's the pic where you're suppose to look.
Click the "Change Background" Link, then you'll see a pop-in window.
Note: If you don't see the change background link yet, then it's not yet available to your area. But Google will be rolling out this feature in your area soon!