Facebook
Seriously though,
Flickr, deviantart
I'm sure you've already got both of those.
NASA's cool too. Really though, there's enough on Flickr that you don't really need anything else. Oh, and the CP picture thread!
Depending on the resoultion of your frame (most of them are crazy low) Facebook might actually be good, as there's some amateur photography groups then sometimes have some really good shots in them. If your resolution is bigger than Facebook's don't bother with it. Also, don't sign up for these, but if you're already on it they can be fun to browse.
Digital Picture Frame
