I'm not an artist and don't even pretend to be one, but I can still appreciate visual images. Years ago, I learned to recognize romance novel covers by the artists. Morgan Kane, Elaine Gignilliat, Pino Daeni, and a bunch of others all became instantly identifiable. (I collected every book with a Kane cover I could find.)
In these days of author self-publishing, it's not financially practical to hire an artist to paint an original cover. Most of us aren't professional photographers, and even with the vast technical improvements in cameras and in photographic editing software, we don't necessarily have access to all the rest of the tools needed to create great cover art.
Stock images are the best alternative for those of us who don't have our own personal photo studio, professional models, and racks of costumes. Unless you're the first person to use a particular image and you pay for exclusive use, it's very likely that someone else might use the same image. (They also have your wonderful good taste.) The same goes for the commercial cover art sites, such as cover model Jimmy Thomas's Romance Novel Covers .com. Your book's cover may end up on someone else's book, too.
I've compiled, over the past five or six years, a representative sample of some of the ways cover art -- and sometimes just certain elements of a composite design -- is replicated on more than one book.
This is the original cover of one particular book, which I purchased several years ago. I'm not sure if the scene is a stock photo or not.
But the cover was changed after I purchased the book.
The couple are models used by cover designer Kim Killion of Hot Damn Designs. (Author Bradley does not credit Hot Damn Designs in the front matter to the book, at least per the current Look Inside preview.)
Author Rice used a professional ebook design firm to format this digital edition, per the front matter of my copy. I assume eBookPrep contracted with Killion's firm for the cover art.
How do I know it's Killion's work? Because I used Hot Damn Designs directly. I selected the specific pose of the models, as well as the stock photo for the background. As you may have noticed, the poses can change minutely, from the tilt of a head, the position of hand, a smile, a frown, a stare, a lock of hair, a ribbon. More can be done digitally as well, including changing the color of a dress or a model's hair, but they're still stock photos.
The street scene with the saloon in the background is a stock photo taken of the Goldfield Ghost Town tourist attraction about five miles from my house.
Mixing stock photos with background with text with fonts is a fairly reasonable route for the self-publishing author on a budget. These covers can cost as little as $150 (or even less) up to a couple hundred, depending on how much manipulating the artist has to do and whether or not they need to buy rights/licenses for additional stock photos.
There are subcontractors who use stock photos to make cover templates. The author supplies title, author name, and any other desired text but has little to no other input.
These covers are even more reasonable for the SPA on a budget; these above are from the Jimmy Thomas site and run anywhere from $50 or so. Yes, I know from personal experience.
Instead of with cover artists like Morgan Kane and John Ennis, I've now become familiar with some of the models and photographers. Jimmy Thomas is ubiquitous, even in a terrible blond wig.
Again, there's a subtle shift of the woman's head, but it's all from the same photo shoot.
The work of Polish art photographer Konrad Bak has become enormously popular; if I spot a new cover that looks like it might be his, I can often spend (it's NOT wasting!) an hour or more cruising through his images on Dreamstime until I confirm the photo is his. I'm almost never wrong.
I have more, a lot more, but I think you get the picture, pun definitely intended. If you use stock photography, you're very likely to see your cover, or something very very similar to it, on another book. It's not because the other author is copying you or trying to make sales based on yours. It's just a fact of digital publishing life. If you don't like it, then be prepared to spend a lot of money on exclusive rights or a private photo shoot or become your own photographer and graphic designer. If that's the way you go, I hope you're better at it than I am!