Thoughts about what makes a good picture book

This was a question that was asked in one of my Facebook groups and I thought it was very interesting. 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 should picture books convey a message? 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 message makes a story a little more heartwarming to read and share with children.