Book covers are an odd thing these days because they are very direct in their translation of what is happening inside the book itself. You can, these days, indeed judge a book by its cover, but that wasn't always the case.
Book covers have been in a constant state of flux. The quality has shifted a lot depending on the market or the budget of the publisher. When I was working as a bookseller, I would often find book covers were either very well done or so poorly constructed that it clearly showed the publishers lack of interest in the book; which meant, to me that is, that this book will suck.
You can sit there and judge me for that, but I am a cover whore. I love a well-made book cover that has a robust design to it. I say this because the object we hold in our hands while reading is our first step into the book's vast world. If the art or design of the book cover is terrible, again, in my opinion, it affects the reading itself. Many famous authors hated their book covers, and you can read their thoughts here.
I adore classic book covers because some of them are just gonzo and delightfully so. Some are just bad but its done so earnestly it comes back around to being good again. Then there are some that are so serious and so seriously bad that you can't help but not pick up the book.
For some wonderful history about the gonzo side, check out this video by Nerdwriter:
I am bringing up the subject of book covers because being a self-published author you often find loads of terribly made covers. These are often the case because the writer either doesn't have the budget to hire an artist (Seriously though, most artists will work with you on a price) or they feel they can make a solid enough cover themselves with Photoshop and stock photos.
I am falling into the latter half of those two fields.
I have been playing around with designing my covers, and not wanting to fall into the 'too serious' category, I have been striving to make something that is overall pleasing to the eye and does justice to the story.
More about that, later.