Mine is a pic of an orangutan, I took at the zoo several years ago. He's big, hairy, and looks tired, I think he represents me fairly well. He has more hair than me, but I have a better mustache.
Thanks!@FluffyGroodle
Go up to the very top of the page and click on your name--one of the options is "signature." I believe more elaborate sigs might require you to be a mod/contributor but I'm not sure.