Instructions for Windows Vista 

To change the desktop background:

1. Open Desktop Background by clicking the Start button, clicking Control Panel, clicking Appearance and Personalization, clicking Personalization, and then clicking Desktop Background.
2. Click the picture or color you want for your desktop background.
   If the picture you want to use is not in the list of desktop background pictures, click the Picture location down arrow to view other categories, or click Browse to search for the picture on your computer. When you find the picture you want, double-click it. It will become your desktop background and appear in the list of desktop backgrounds.
3. Under How should the picture be positioned, choose to have the picture fit the screen, tile, or be centered on the screen, and then click OK.

Note: If you choose to have a centered picture as your desktop background, you can also have a color background to frame your picture. Under How should the picture be positioned, click Change background color, click a color, and then click OK.

You can make any picture stored on your computer (or a picture you are currently viewing) your desktop background by right-clicking the picture, and then clicking Set as Desktop Background.

For Apple/ Macintosh computers, please see the instructions at
How to Change the Background Picture on a Mac