For
Windows users: Right click on the image, and a menu will appear. Choose "set as
background" or "set as wallpaper".
For
Mac users: Move your cursor over the image, hold your mouse button down until
you see a menu and select "Save This Image As." In the dialog box that pops up,
choose where you'd like to save the image. Go inside your apple menu to control
panel, and select "appearance". In appearance, select Desktop, then click "place
picture." Select the image file you saved and click "choose." Select "position
automatically." Click "set desktop" and close the window.

1024x768
screen size (may work with other sizes)

Al Green Wallpaper, 800x600 screen
size

Al Green Wallpaper, 1024x768 screen
size

Al Green Wallpaper, all sizes