I am totally puzzled. Look what happened.... I registered on xenForo today and the default Avatar I got shows a picture of a woman. I couldn't believe my eyes, because I recognized the woman on the photo: she is somebody I know personally!
I see the Avatars are taking from Gravatar (?), but what a coincidence! I would almost think... this is a sign!
If you register and your email you use is used by gravatar, it will try and get the primary image for that email from gravatar.
And what I really like, is that if you update your primary image at gravatar.com, that after the cache is updated on their side, it shows the new gravatar automagically on xf No need to do anything!