Shutterstock's CEO acknowledged the tip of the problematic iceberg in the company newsletter. It's not only a matter of creation, but a matter of licensing. In current legal theory, the producer of the art owns the copyright and can license it.
But who's the producer in the case of AI-generated art? The person who created the program? The person who used the software?
What few decisions that have come so far are things like the Monkey Selfie case. The U.S. Copyright Office subsequently came down with the decision that if a human wasn't substantially involved in the creation of the art, then they cannot own the copyright. They added that non-humans cannot own a copyright.
It's a freaking mess. Right now, I would suggest you be very careful with the cover artists you may use for your book covers. Because money follows success, and if your book is super-successful, then you may be courting potential legal trouble when everyone wants a slice of your pie.