Are you tired of the pictures and patterns Microsoft provides for the Desktop background (also called wallpaper)? Using Picasa, you can transform any image in the library into your new Desktop background.
Select a photo thumbnail in the Picture Library or open the image for editing.
Choose Create > Set as Desktop. Click Yes in the Confirm dialog box that appears. The image becomes the new Desktop background/wallpaper.
In the Display control panel, the file is named picasabackground. Each time you create a new Desktop in Picasa, the existing picasabackground file is replaced.
If you later want to restore your previous Desktop or select one from the background images supplied with Windows, open the Display control panel. Click the Desktop tab, select an image or pattern from the Background list, and click Apply. If the image is satisfactory, click OK.