About 12 users out of 2000 on my client's site have avatars that they didn't upload. They seem to be random images, they're not present in the imported users CSV file. Beside each of them there's an upload dialog and underneath that it says 'No local avatar is set. Use the upload field to add a local avatar.' There is no way for the client to delete these images.
Where do I look in the database to do this for them - and why would random images appear as avatars in the first place? These images don't exist anywhere else on the site (which is not yet live).