There are many sources of images, but one of the dangers of using stock photos is that they can be sold multiple times. If it is a good image, chances are that someone else will have used it. As a result, spotting re-used images and designs has become a kind of hobby for many people. It involves perception and memory, and I play the game myself.

As an example, here are two covers I spotted
a while ago that use the same base image.

I first got into the hobby after reading Dark Echo, then spotting two other books with similar covers, using the same core image:

The three above are published by Hodder, Penguin, and Harper, respectively. This is obviously an issue for all publishers, including big trade publishers. It's also why good cover designers always make an effort to check if stock imagery has already been used (e.g. by doing reverse image searches).

By the way, I’m sure they're all great books, I'm not saying anything bad about the interiors. The authors probably had no say in the covers either, since it is a rarity in trade publishing for the author to be involved with those decisions. At most the authors are usually just asked for their blessing and to share it on social media when they see the final version. The actual cover is likely created by a design team based on genre expectations, trying to make the book look like others in the genre that have sold well in the past – hence book covers often go in cycles, and you will see lots with similar designs. I see this with many genres – psychological thrillers, chicklit, serious women’s fiction, epic sci-fi, horror, crime etc. We see repeated fonts, layouts, colour schemes, and effects.

I decided to write this post because I spotted some more covers yesterday (I can't share them because they were works-in-progress that my fellow author hasn't released yet, but the central girl on a motorcycle had been used on another post-apocalyptic book cover which I just happened to have on my Kindle already, making it easy to check the match! Both were cool covers.)

If you are interested in this phenomenon, or just in book cover design and trends, then here is some further reading.

Updates since the original post:

Different stock images this time, but very similar overall styles. It's likely to be a genre thing rather than one designer aping another, but it's often surprising how similar the end result can be.