You can use any username you like on Gravatar, it does not have to be your AB one. Once your account on gravatar is created you can upload a picture you have saved on your PC or provide a link to an online one. Make sure you set the rating as G for it to be usable here. Then go to your profile on AB, select to activate avatar and click submit at the bottom of the page.
Just follow the instructions on the GRAvatar website. There'll be links to click, and a choice of selecting a file from your PC or entering the address of something you've found on the Web.