I’d get more into the topic, but I haven’t got time to plumb the depths of Amazon. Instead I recommend that you do. Look at the genre you picked for your story, and see what the bestsellers are (taking out authors like RR Martin and mega-bestsellers, who can sell on the author’s name alone) and look at their covers. Keep in mind that most ebooks will be viewed at a very small size of image initially, and design yours to look good at thumbnail, then full-size. Stick to the recommended proportions. A square cover image will scream amateur in the bookshop, unless it’s a children’s picture book…