Again, I am not a lawyer, but the way I understand it, you own the images and therefore own the copyright, so you could sue someone if they start using the art somehow. Again, though, I am not a lawyer.
I think you are focused on the wrong things, if someone steals your art work it means you’re doing a great job. If you really care, then spend $10,000 on a copyright lawyer to set your images up with legal protection. Short of that and people can probably steal your art work and get away with it.
They could even steal your art work, copyright it themselves, and then you are the one that could get sued. But again, this is all the wrong focus and shouldn’t be a concern until you’re making millions in book sales!
Well, what I actually care about is the possibility to use these images (after a certain amount of editing) as book covers and sell the books online. The risk of being stolen is secondary, and to me it’s sufficient that those covers are associated to my name.
There are already good answers, but for detailed information you can also see what US Copyright Office initially thinks about AI generated photos. I suppose the legal thinking is still evolving, but for now it might be best to have some additional human input to make sure it can be copyrighted in the U.S.
So, in addition to terms of service, copyright offices around the world may treat “AI photos” differently until there is a unified framework.
Easy way to make it “non-AI photo” is probably to modify it.
I don’t live in the US, but anyway copyrighting these images is not my primary concern: I usually self-publish my books on Amazon KDP and I suppose that there is always a date associated with the publication, that might have its legal impact in case someone would steal them.
As for your last point, all my AI images are darkened because they depict human profiles (more saturation, more shadows and contrast to make their faces unidentifiable, just to be sure), I don’t know if this is sufficient to make them “non-AI”.