stock art!
When I make my own book covers, I use a lot of stock photographs for the covers (you can see one such example above).
I’m not even remotely the only one who does that.
Found via The Passive Voice, here is a great article about the use of stock images for covers, and how the same images tend to turn up over and over again in book covers.
I have to admit that an infrequent hobby of mine is to walk through a Barnes & Noble and spot all the books whose covers have images I recognize from stock photo sites. š
-JM