Mine is an image of my old superhero alter ego: "HatMan: Avenger of Boredum (and bad speller...)"
Originally, I was MadHatter (being both cheerfully insane and, due to premature baldness, never seen in public without a hat). But HatMan was created to have fun and adventures during conversational lulls. Eventually (because I liked it more and because MadHatter was used by a few too many random jerks, which caused me to get some angry and confused PMs), I started using HatMan full time.
(The long version of this story, if anyone is interested, is in an old
"Where did you get your nick?" thread.)
When these shiny, spiffy new boards were created, I decided that the momentous event deserved a new avatar. So I "created" an image of HatMan, the cheerful superhero with his trademark beanie copter. By "created," I mean "stole a picture of Robin by a professional artist and then hand-drew a silly hat on him with MS Paint."
It's been my avatar ever since.
