I could be wrong, but it appears that a user's profile picture is tied (through Gravatar) to the e-mail he uses for notification. Is that the case? (Changing the notification e-mail certainly seems to change the only e-mail address visibly associated with a profile.)
If so it seems an odd linkage, since the mail one happens to choose for notifications is certainly determined by a variety of generally private (and typically irrelevant to SO) considerations, while the picture is (obviously) very public and particular to SO. Shouldn't there be a distinct setting for "picture e-mail" in the user profile for Gravatar to use?