Let's see to fix that then,
Should be line 287 - function pia_main() - FIND and DELETE this line.
AND user_avatar_type ' . $this->db->sql_like_expression('') . '
SAVE and close that file.
Automatically at the first hit to the board the logic will (should)
populate all of the remaining avatarless users in one shot..
Edit: you should then hit submit in ACP to let it fill out those users.
The main logic called on a per board's hit only runs if there are logged-in new avatarless users.
The ACP instead forces that logic to run with no conditions, just the submit button.