I read quite a bit (a lot) of young adult fiction, as I am a young adult, and over time I have noticed a recurring theme in the covers of the books I'm reading. Side note: I can't really speak for other genres, although I imagine adult fiction is fairly similar. Nearly every single time there is a cover with a photo of a person, they are "perfect", skinny, unblemished and photoshopped when you look.
But you so often don't. We are used to questioning what we see in magazines and in adverts because there has been so much coverage of that, I think we need to question our book covers too.
They say 'don't judge a book by its cover', but we undoubtedly do. This is natural; if the covers were all the same we would judge it by the title, you have to judge it on something, it's how we choose things. But the interesting thing is, there is a side-effect of this. The images of people. (Almost) Every human on the cover of a book is perfect, beautiful, photoshopped and unrealistic. The designer wants us to buy the book, and so therefore they make it as "attractive" as possible. Why is it less common to question how beauty is portrayed on a book cover than a magazine? Is it because we shouldn't judge the book by its cover, that the cover doesn't matter? Because it does. Everything does.