Generally, I just find images I like at random, and play around with them until they fit.. I've been using this one for a while, though..
When I used to play around with sigs, I would create avatars to coincide with them, but I've been far too lazy with that sort of thing to bother, anymore.
Yeah, signatures can be pretty damned annoying.. On several of the sites/games I've modded, people pushed the limits on things whenever they could.. And then when they got hit with forced limits, they whined that a couple of pixels would force the image out..
Too many people would have signatures longer than most of their posts..