>>9On second thought, there may be a way.
a) Make sure these problems aren't caused by database limitations. If they are, and not simply solved by exchanging an int32 to an int64 or something, better roll back to the old site and restart the software evaluation. Everything else and you are in a world of pain. You may like that, but make a separation between gurochan fiction and web admin reality.
b) Copy the original images with their original names over to where they belong. No need to delete the broken images beforehand, you can identify them later by the shorter names, but check and verify that.
c) Extract the linking between board name / post id / image name from the old database. That's unique, there's only zero or one image per post.
d) Put the old image names into the new database where they belong, based on board name and post id. That depends heavily on the database structure, of which I don't have any idea, and you should definitely know what you're doing. Also, make sure the image number counter works correctly and starts behind the highest image number.