Important! If you attempt to register and do not get an email within 5 minutes please check your spam box. This is especially true for Microsoft owned domains like Hotmail, Outlook, and Live. If these do not work please consider Gmail. Yahoo, or even AOL email which works fine.
It's already set to do this. User CP -> Edit Avatar, scroll down, and you can link to or upload an avatar. (I think the link uploads a copy either way for security reasons.) You would never have a direct link as someone could change the file.