Under Mountain Lion, the monitors have a choice of diverse photo sources, including permission to retrieve photos from National Geographic. A quick tip for a sublime watch screen!
From the menu Apple, click on Preferences System... then on the module Desktop and screen eco..
In the left column, choose for a slideshow mode that suits you and then select National Geographic in the list of sources. You can also choose between: cosmos, air and patterns of nature.
So old-fashioned a screensaver? Not so sure...