You can also use the same cover art and just repurpose it a little, as you can see here.
How about book covers that try to piggyback off of a successful book? Click here for an example.
Sometimes people just have the same good idea. Click here for an example.
I've only had one cover that was pure stock photography (in fact, my first three were all made especially for the book, and two others have at least mixed images). Luckily, I've never seen this image anywhere else - have you?