Novelish has a way cool article on when two different books use the same, or similar, stock cover photography. It’s full of comparisons and source images that have been tracked down. The most amazing thing is that the author has been able to track down all these covers.
Go into any bookstore and you’ll see photos on covers everywhere. I’m not sure whether publishers think photography sells books better or they’re just too cheap to pay an artist, but it seems to be a growing trend in cover design.