My understanding:
That happens often, in older threads on web forums all over the place, when the pictures were linked to some photo-hosting company (like Photobucket). When the hosting company ceases to exist or do funky stuff (like Photobucket did, IIRC), then the photo ceases to exist in the old thread.
It shouldn't happen if the photos are uploaded directly from a user. If it does that, then I think it would be a problem with RCU.