Informally we usually call the person involved in the art-making process an artist. In your example, there is another person in the loop who is making the art.

I don’t think anyone is arguing to AI personhood yet, so it must be the user who is the artist.

If we want to add extra conditions, that seems fine, but someone else will have to define them. If it were up to me… I’d probably end up being called a gatekeeper.