The biggest challenge as a cover designer is explaining this concept: the root of all sellable covers is having a clearly distinguished genre. I cannot tell you how many writers have a symbolic, artistic idea for their stories (“I imagine Jill on a train, and on the right side of the train there are dragons, and on the left side there’s a mysterious-looking man”). Unfortunately, those types of covers rarely sell (if ever).
Before I interned at North Atlantic Books, a publishing house in Berkeley, I was guilty of doing the same thing. After I wrote my first book, I wanted to have a silhouette of my characters going through a mirror in a dark wood. I’m glad that I never put that cover on my work! Designing and marketing books is not about being creative. It’s about producing something geared to the book market.
Writers, being the creatives that we are, tend to envision a scene from our books, or invent some sort of symbolic reference that we want to represent our manuscripts. The problem with this is that writers are generally not in marketing or sales, they haven’t taken hundreds of hours of design classes, they don’t have fine art degrees, and they haven’t researched the publishing markets to discern sellable trends.
The best covers for sales merely act as signs to catch your audience’s attention, and many of them don’t relay anything about the story at all. Often it’s merely the font type, the color scheme, or the general layout that grabs attention (and then your back blurb does the rest). If you look at particular genres, you will start to notice repetition of design elements. That’s what gets the fantasy reader, or the thriller reader, or the true crime reader to pick up your book.