Heh - the YA author Patricia Wrede says that for children's book covers, at elast, the artist has to get it right. Adults will ignore the main character painted with red hair when it's described as blonde in the book, but with kids you'll get a ton of letters in pencil on lined paper that say "youg got the hair color wrong and you suck." :)

I was at a con and attended a panel about cover art with David Weber on it, and he said that he left the cover art up to the publisher and artist, unless they came and specifically asked him questions, because they knew what sold. And that once he mentioned to an artist that they got the ears wrong on an alien that he'd written, and the artist said that he put the ears down lower on the skull because the character was supposed to be friendly and sympathetic and if he'd put them higher, he'd have to change the shape of the skull, andhe wanted to keep it similar to human, for the friendly-and-sympathetic effect.