@flev
Here are a couple of things to try.
Depending on when this loss happened, your test or beta instances of Canvas may contain the links to the existing avatar, including the URL. You may be able to transfer that information back to the production instance. The test instance is almost a month old at this point, so it may miss out on recent changes but the beta instance is updated every weekend, so it may be gone already. If you can get it from there, I would save it somewhere as it sounds like there was an issue with your system that overwrote old photos.
If a student uploaded a picture, then the picture itself will likely be available in that user's My Files in the profile pictures folder.

There may be more than 1 image for a student and knowing which one they were using can complicate things. Since a student cannot choose an existing file for their avatar (they have to re-upload it), I would guess that the profile-2.jpg in my example was the most current version.
Another option is to notify students that they would need to reupload their images.
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