There's a standard for how much it has to be modified, but yes, if you provide a significant creative addition to a public domain work, it is copyrighted. This is how orchestras that play old (out of copyright) scores get copyright to their renditions. If you copyright the midjourney image and (hypothetically later) there is some legal dispute, you won't like the resolution there, as the damages will be considered willfully inflicted on the other party. I think this largely moot, personally (because of the number of people who will modify works to make them legally derivative, people aren't going to go looking for stuff that should be free much, but that's just a guess).