To use an avatar, click User CP at the top of any forum page and click on the Avatar option. It should be self-explanatory from there.
I will make an avatar for you if you like. The advantage of this, as you may be able to see, is that you will get exactly what you want and it will be the proper size with no energy on your part. I made the avatars for Unix X11, Krevinek, and of course myself, and I'd be happy to do it for you.
Send me a private message if you'd like me to make you an avatar.