I suspect this would be solved by visually comparing the two images and noticing that they are one in the same. Than you can just see which had their hash published on the block-chain first to see which is the real one.
Just identifying the images are the same doesn't prove which was the original though. The hashes being publicly available is the key, because you can easily tell which came first.
Hashes don't verify the date that something was created. The blockchain provides a date for the hash, but it provides no guarantee that the hashed item didn't exist long before when it first appeared on chain. If I simply modify the image metadata to say that my knockoff photo was taken at an earlier date and flip some bits, I can still claim that mine is the original and that I didn't submit it to the blockchain when I created it.