From your AppleTV menu select Photos then .Mac.
From the .Mac menu select Add .Mac Web Gallery
Input your .Mac username and select the Done button.
Now select your .Mac username name from the list of .Mac Web Galleries
You will be displayed a list of the .Mac Web Gallery Albums for your username. Select a gallery.
You will be displayed a slideshow of your select Web Gallery.
To set the settings for your Slideshow you can select Settings from the Photos Menu.
Here you can set Time Per Slide, Music, Repeat, Shuffle Photos, Shuffle Music, Ken Burns Effect, and Transitions.
To set this Gallery as your Screensaver select Screen Saver from the Settings Menu.
Select Photos from the Screen Saver Menu.
From the Choose Photos Menu select .Mac.
Select your .Mac Username.
Select the Gallery you would like to use as the Screensaver.
The Screen Saver menu will now indicate which album you selected.
Wait for a few minutes and you will notice your .Mac Web Gallery is the new Screen Saver!