I just use Photobucket. Just make a copy of the picture and then select the edit feature for that picture. You will see a a couple of different size options. Just click the one that says avatar. If you have to crop the picture I always do it on my computer before I upload it to the internet.
I found the edit tab and hit avatar like Jones said pasted it and VIOLA an avatar. (helps to be logged in to the account you have at pb LOL) Only glitch, was had to do an update for their resize shop, couple of seconds and done.