I have hit exactly the same issue as @jrgen highlighted and fear that the only option might be to delete all the images transferred from vBulletin and then make it very clear to people that they should only use images for their profile and background that they are happy being potentially publicly associated with their username. Because profile images have URLs like this: https://forum.example.org/user_avatar/forum.example.org/chrisc/120/26_1.png (background image URLs are not so bad since they contain a random string).
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