I'm not sure if this has been posted before but I doubt it. Some of you may have seen this site before. But it's called Digital Blasphemy.
It has some of the most unbelievable pictures I've ever seen...some can be used as cool background for sigs if that's what you're looking for. So far my favorites are Tropical Moon of Thetis, October Flame, and Aquamarine...but I'm yet to see the entire site so I'm sure there's many awesome ones I've missed.
This post is pretty random, but it's a very cool site and I advise you check it out. They make damn nice wallpapers.