Add a step between "no gravatar account" and "default gravatar sent back", using Google Images Search Service + 2 bonus usage
This plugin takes place between the gravatar picture and the default avatar return. If an email is not associated with a gravatar account, a Google Image request is sent to grab the first best relevant face picture of this member.
When a google image photo have been chosen, the user will receive a mail with 3 links, 1st to delete this avatar if not relevant enought, 2nd to refresh it, if its email is not associated with a gratavar account, 3rd an invitation to create a gravatar account.
Also, you can add you own avatar by uploading your picture, easily !
Then, you can use a gravatar cache to keep all avatar files on your server and, limit the HTTP request on gravatar.com
Here comes a (french, just watch ;p) demo video, 15mn length. (video = version 1.8 !)