Simply go to gravatar.com, log in there with your AB email address and select a new image.
With any picture that you fancy on the internet right click and you should get the options to view and also to save the image. You then have a choice to save the image to your computer and then upload it to gravatar, or, when you view the image, copy its URL from the address bar and use that option in gravatar (i.e. misses out having to do it via your computer).