I spend a lot of time thinking about my book covers and choosing the best option. Like everything else, I often overthink it, but in general, a cover must give an overall feel of the story inside. For example, many of my books have a dark, sinister vibe that is captured on the cover. These days, I write about criminals, so you wouldn't expect to find an image that is bright, cheery, and fun. It wouldn't make sense. It would be misleading and send the wrong message about the story and the characters involved.
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A perfect example would be my Hernandez series. The protagonist is sinister, powerful, and dangerous. Both the covers in this article will show you the connection to this theme. I lean toward ominous imagery. I also have bright colors that are eye-catching and attractive against the black background. This is what captivates your audience.
In the end, each author has to decide which cover best represents their book while at the same time considering the marketing aspect and what will sell. It can be a difficult balance but is necessary when publishing a book.