First you need a site that allows images to be displayed on other sites (hyperlinking) such as www.photobucket.com
And please do get your own account there or somewhere else because otherwise you're stealing bandwidth by making others' images load.
Ok, then get your image URL and simply use this code:
(ends with the > actually.. I dunno why it's putting that ; at the end)
(actually you can leave off the quotes around the address and it still works) and place this code in the 'picture description' box in your profile. It might work in other boxes too.. I haven't tried. Oh, and nothing too large please or people will hate you :)