Claiming ownership for a picture is something that would need to be run through the admin and moderators, in order to prevent someone from "taking" a bunch of unrelated pictures. This is something that could definitely be possible, and a few people might enjoy to be able to add all of their older pictures to their own gallery. I added it to my TODO list, I think there's potential for that feature. Thanks for the idea.

A year later, I'm happy to report that the brand new IOekaki v1.6 does allow you to claim back your lost pictures! Hit the "Claim picture" link available at the bottom of each image, enter a title, then hit the button. One of the mods will review your request and approve it if everything looks fine.

You can use this either for pictures submitted on the old oekaki board, or for files that were restored manually and for some reason aren't linked to your main profile.