I usually use images that represent my characters, and for most of them, I've already found an image that suits them, although for a few, I've found the image and made the avatar/signature right then.
Most of the images I've found for the current one seem to suggest the character(from the series he's from) suggests he is at least a little crazy. Could be wrong about that, as I haven't seen the show yet.
The only character I can't seem to find a good image for, would be Luna Moreau, the Prince of New York in my writing. You'd think it would be easy, but not really.