Just google image search what you want, and in the options select "icon". Most of them are the correct size. Or create a photobucket account, upload your pic and resize it to 50 x 50
Quote by ThinLizzyFan
I love you

Who's in a bunker?
Who's in a bunker?
Women and children first
And the children first
And the children
Quote by fg-433s
How do you change them???
Wait ... what?

How did you get your current avatar?
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
Give James Cameron more money than he'll ever need.

However, for a more serious answer...Look right underneath the thread title in the bar just underneath the ad at the top of the page, there should be a "Control panel" option, in there, find "edit avatar" and voila, browse through your pictures, pick which one you want...and there you have it.