Book covers are the most important marketing tool an author or publisher has at their disposal. Think of them as mini-movie posters, it's the same concept. You have ten seconds TOPS to draw in your reader or viewer and convince them they want, no need, to read your book or watch your movie.
Celebrities help, flashy review blurbs help. But those aren't always available to new authors. So what's a writer (or publisher) to do? Hit 'em with your best shot. When working with a publisher, it's important to have a concept ready for what visually represents your book. You're the creator, you know your book intimately. Think about what images floated through your head while you were writing the book. What one or two key images or action sequences rise to the top?
Although publishers will work with professional designers and artists on your cover, many of them (like us) are interested in your visual concepts and ideas for the cover. There are even times when authors have a special artistic talent and can contribute graphical elements to the story that they've created.
Covers, like a book's positioning in the market and genre/theme, follow trends. Search for titles that are similar to yours. Check out the best-sellers and their covers. Note the ideas, themes, colors, design and look and feel elements that you like. You can share these with your publisher to give them an idea of what you feel represents your book.
Romance titles, for example, have gone from the Fabio-clinch days to more love-based visuals to chick-lit high heels...and back again. Read more about romance cover evolution in this illuminating Publisher's Weekly article. Regardless of genre, each cover must distinguish itself from the competition but still work within a brand aesthetic—all while enticing readers to give something new a try, but still represent the story readers expect. Whew!
All this is to say - you don't have to be an artist or designer to have a great idea for an eye-catching book cover. All you have to do is close your eyes and let the images speak for themselves.