I'm noticing this happening more and more. Occassionally I've come across it in traditionally published book generally books published by the same publisher and in the same vein.
But I find it happening a lot in indie pubs now. I've noticed several images being used multiple times on several differing titles. I tend to notice it if I own a book using the same image(s)/model(s) and see it on the 'new releases' list. It causes a rubber neck reaction and it takes a second closer look to see that it's a different book altogether and I'm not going
like I originally thought.
I figured that the publishers were running out of new images to use and were recycling those they had the rights to to save themselves mullah.