Children's books often have different illustrators and writers. I think that images in a book can have as dramatic, if not more, an effect on how a child interprets the story and imagery. If done properly, the images can serve to improve the conveyance of the message and increase overall enjoyment of the book but, if done improperly, can skew the message or reduce the coherence of the book. Many books with simple sentences, strange rhyming schemes or made up words benefit immensely from pictures.
The first author that comes to mind when i think about this relationship is Dr. Seuss, a man who was neither a doctor nor a Suess but he wrote some darn fine children's books. Half of everything he wrote was ridiculous, nonsensical rhymes and made up words that were simply fun to say and the images that the provided along side them really help to make a little sense of these strange phrases. The pictures helped you to understand what a Wocket looked like and what color a Truffala tree is. They set moods and pretty much accounted for all of the actions that took place in his books.
I have also seen a huge amount of books, both for children and adults, where it doesnt seem like the illustrator actually read the book before designing illustrations. this can get very confusing when the picture has almost nothing to do with what is supposedly happening.