Y'need to register at gravatar.com using the very same email address you use for AB (username does not need to be the same). Upload images either from your comp or the internet and rate them G. Choose the one you want as your avatar by clicking on it and then clicking on confirm. Return to AB, click on your profile, click on "edit user details", tick the box to enable avatar and then press ctrl F5 !
go here http://en.gravatar.com/ and follow the instructions it doesn't matter what username you use so long as the email address you use is the same as the one you use foor AB (you can check this in the AB my profile edit user details) when you have got your avatar go back to the edit user details and you should have a box labeled activate avatar clic and that should be it., having said that people have been doing this and still had trouble