Here’s an experiment. Go to Amazon and find 10 comp. titles for your book. Look at the covers. What do you notice? There are probably patterns of design elements. If it’s fantasy, you probably have a unique serif font on the cover. If you’re looking at thrillers, you probably have bold, condensed serif fonts with blue, yellow, black, and red color schemes. Historical fiction has a lot of back-view characters facing cities for images. There are patterns for every genre, and readers are subconsciously aware of these patterns when buying.
It has always bothered me to have a picture of the character on the cover of a book. I want the character in my imagination to not be changed based off of someone else’s interpretation. However, it doesn’t stop me from buying books with characters on the covers because I know it’s a pattern in fantasy, and I like reading fantasy.
Sure, it’s nice to have an amazing work of art on the cover of your book, but if it doesn’t fit into the right market, your audience isn’t going to look twice.