You go to photobucket or google images or any other picture site, search up what you want, find it, and save it to an easily located place in your computer. Then, here you click "Go to Control Panel". Then, you click "Settings", open the "Edit Personal Info" bar, and below the "About Me" section, there's an avatar selection box. Click "Browse", and search through your computer for he picture that you saved, and click it. Once you've done that, click "Save Personal Info". It might not show up at first, so just refresh the page a couple times.
Yes, I copied and pasted from CG Community and pasted it here for easy access to the cheat.