Have you put it on the web somewhere and got the url to the picture?
www.imageshack.us if not. You then put this address in the avatar link I gave instructions about earlier in this thread.
Get it on imageshack, and if you get stuck, send the url you've got from imageshack to aid in further instruction.
PS. If this is a close up of her ass, that's probably too strong for an avatar. Ask the mod astroglide if you're not sure if it's too dodgy, is probably the soundest approach.