[stonemetal12]

>To be fair, a prompt fed into a generative tool _could_ be considered an artist's creative expression.

Depending on if the prompt met other guidelines for copyright, it would be pretty uncontroversial to say you own the copyright on the prompt.

Copyright on the picture, is about as assignable as if you invited ten painters over to your house and read the prompt as spoken word poetry, then received one painting at random. The fact that your prompt won't reliably produce the same picture suggests that you are not in control of the artistic choices made, and therefore have no claim to the copyright.