Machine generated works cannot be copyrighted, only human-created works. The part here that has yet to be determined is whether or not AI output is considered "machine generated". An argument could be made that writing a prompt is sufficient control to consider the output human-made for the purposes of copyright. If that's the case, then copyright will go to the human who wrote the prompt (or their employer).
This will, at some point, have to be decided in court. Right now, the only answer is "nobody knows".