Add some image services in your WordPress site discussions. Add a step between “no gravatar account” and “default gravatar sent back”. Make comments system automatically search and set the commenter’s image.
Using Gravatar Google Image you can add one more functionality between the gravatar picture and the default avatar return. While commenting when if any email is not associated with a Gravatar account, a Google Image request is sent to grab the first best relevant face (from Google image search) of the person who commented. And then it emails the commenter and makes him know about the image used and provides him the ability to remove or edit the image with an option to use same image permanently through Gravatar. See how it looks like.