an illustration or a graphic representation or words. Color is important. I do not like photography on book covers. I like to reserve the right to imagine the characters myself. A drawing lets me. It's not so definite. A photo demands that this is what Susie looks like and if I've imagined her differently or don't like how this model looks, too bad.
I buy books because of covers. I wonder about the cover as I read. One of the reasons I like physical books as opposed to ebooks is that I can go back and look at the cover whenever I want -- okay, okay, I know you can do that on an ebook too, but it's not the same as just holding the cover in your hands and often the printed book has a different cover than the ebook. The fonts, the color, it all makes a difference to me. Have I liked books that I hated the cover of? -- of course, just like I've been devastated by bad books in great covers. When I listen to books on my iphone I find it very satisfying to hear the story unfold while staring at the cover.