AB does not copy the avatar to its system - it is linked from the Gravatar site - and is expressly forbidden by Gravatar to "copy, store or modify User Submissions" by the Gravatar Terms of Use.
Another of the Terms of Use is that "you hereby do and shall grant to [Gravatar] a worldwide, perpetual, irrevocable, royalty-free and fully-paid, transferable (including rights to sublicense) right to perform the Services (e.g., to use, modify, reproduce, distribute, prepare derivative works of, display, perform, and otherwise fully exercise and exploit all intellectual property, publicity, and moral rights with respect to any User Submissions, and to allow others to do so)."
In other words, you relinquish control of the avatar image to Gravatar (and the rest of the world) from the moment you submit the image.
http://en.gravatar.com/site/terms-of-service/