Designing movie posters is an art form that is extremely demanding. Huge sums of money ride on getting that poster just right. And if you think about it, they’re often marvelously well done.
Why does that matter to authors? Because a movie poster is much like a book cover. It must represent in a single picture with accompanying text what a movie/book is about and motivate people to spend their money. It’s also typically done in portrait mode just like covers. Learn how to design a movie poster well and you’ve learned how to design a good book cover.
Thomasz Opasinski is one of the world’s premier movie poster designers, having creating hundreds of them of major-release films. And Adobe, through their Inspire magazine has posted an article on his “Tricks of the Trade,” including some three-minute snippets of useful techniques and an almost-hour-long webinar. You can find that here:
You can find illustrations of his work here:
Click on the arrow on the right to view each category. They are most impressive. Perhaps you can find an idea for your book cover there. Remember the first and most vivid impression people will get of your book is that cover.