Hopefully the image owner can try out this platform. An uncensorable, unmanipulatable blockchain may just be the exact thing he needs. Imagine staking your claim in your work with unmanipulatable proof of time.
While copyrights exist to protect the original creators against abuse by others, it needs to be subject to fair use policy as well. Your work does not directly profit from his artwork, as it serves to provide some imagery. Although it is a mistake on your behalf for failing to credit him, The way you handled it is commendable.