I don't like covers with photographs from the movie recently made from that novel. I have a copy of I am Legend with Will Smith on the front, and the main character from the actual novella version of the story is as totally, completely different from him as can be imagined. I don't like covers with photographs of people at all, really. I have a couple of books in my library that are dated by the hair styles in the photos on the cover, even though the stories inside are still timeless enough.
I prefer an artist's conception, a collage, something random or artsy. Not a famous painting, unless it has something to do with the book. Several of the novels we ordered for the students from a particular company have famous paintings on the covers which have absolutely no relation to the book, and the students are always saying, "Is that Maisie?" No. No, it's not. It's some random Matisse painting. Arrgggh!
Oh, and also, please, no spoilers on the cover. One version of Roll of Thunder, Hear My Cry shows the children huddled on the porch while the farm burns, which gives away a climatic plot point and doesn't happen until the very end of the book.
--Q