I can't say I can recommend an actual site that hosts that kinda stuff, but they prolly do exist. But if all you want is a picture or simple logo, you just need to do a Google (or engine of choice) image search. You would also either need to refine the search to small items, or save something to your computer and shrink it to size. Or you might choose to be an avatar pirate, and visit all kinds of forums (not necessarily guitar) and save someone else's.