They aren't even contradicting in my view. Rather they say the same thing. The saying you shouldn't judge a book by its cover implies that most people actually do judge a book by its cover (otherwise, you wouldn't have to explain it). And the reason people do is because a picture is worth a thousand words. We're visual people. That cover expresses a lot, a 1000 words worth of stuff. So we use it to judge the book, even if they're unrelated.