The importance of visual elements must not be understated, even in the case of a nonvisual medium like a book.
Too often you see half-assed shitty covers, but this is a mistake.

It's not a matter of "baby want picture book," it's a matter of immediate impression. We say "don't judge a book by its cover," but the cover is an enormous part of how readers decide to read YOUR book.

I would go so far as to argue that besides your cover art, you should have illustrations of your principal characters. Even if AIslop, that's fine. AIslop can be a workable placeholder.