This question was asked in one of my Facebook groups and I thought it was a very interesting question. For me, the most important part of a picture book is that pictures and text combine into a really exciting or endearing story. There has to be a synergy between the two. Pictures are, of course, hugely important, but if the relevant text isn't there - with some sort of interesting or humorous storyline (and repetition is fine for young children), then it'll just be a collection of pictures. And if an interesting text is there without pictures to fire the imagination as well, then that's no good either. And do picture books always need a message? Well, I think most authors who write picture books want to convey some sort of positive message to children - I know I do - and I think a story with a message is always a little more heartwarming to read and share with children.
To illustrate my point - UK children's writer, Julia Donaldson, is very well known for her rhyming picture books, such as the Gruffalo (a huge, terrifying monster) and Room on the Broom (about a witch who encounters various animals). The text is funny and humorous, but the pictures bring the story alive and make the text and the message doubly effective. You can't divorce one from the other, they both have to work together in harmony.
What do you think? What is the most important part of a picture book for you?