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WALLPAPERS
PC
Right click on the image and 'Set as background' or 'Set as wallpaper'.
MAC OS X
Drag the image onto your desktop. Go to System Preferences. Then go to the Desktop icon and open it. The drag the image into the well.
MAC OS 9
Drag the image onto you desktop. Go to your Control Panel and select Appearance. Click 'set desktop' and choose your new desktop.