It's all built on SQL, so Bobs (who likes that sort of thing
) could scan the database for all picture references, and give us a list of posts. We would have to then grab pics dump them in the attachments folder, and poke the database again to pick them up from the right place.
It's the grabbing part that's the real pain. We can't be the first to have this problem. I'll have a look for an SMF extension that manages pics.