From an old thread someone linked to: @dorkusmalorkusmafia. We were still on vBulletin back then of course, long before Discourse. So how does he have an avatar? I seriously doubt that the avatar Greasemonkey script that some people ran 10 years ago would cause an avatar to carry over. Weird.
Best guess is that, at one time or another, the standard VBulletin avatar upload section of the user control panel was active, even if those avatars weren’t displayed on the forum pages.
I seem to remember that that profile pic (only visible on your profile page) became your avatar when we swapped over, if you didn’t have an avatar.
From my vague recollection of the Dope on VB, that sounds most likely.