Do you use Windows 7? If you do, you can download and install the latest Trekking the Planet theme. This theme includes Eleven (11) wallpaper photos. They will automatically be rotated every 30 minutes.
Instructions: If you run Windows 7, right-click on the icon for the themepack above and then select either Save Link As… or Save Target As… (depends on your browser). If all else fails, you can always select a single photo from the list below (see next paragraph).
If you don’t have Windows 7 or are having trouble with the instructions above, you can still select one of the wallpapers below. If you need help setting your Desktop Background, you can find instructions here.