What makes a good picture book for me it's an author that can write text "visually" and an illustrator that can visualize text.
A picture book might be opened a thousand times over by a child that has yet to learn to read. The words may have been read to them by a parent or teacher but until they have memorized each line, they are telling "themselves" the story through the illustrations on the page.
I love the "simple" yet not, pic book, "What Are YOU So Grumpy About" by author/illustrator Tom Lichtenheld. An example of text, "did somebody leave the toilet seat up and you didn't notice?" hummmm but when a child see's the picture of the huge toilet and only the boys feet sticking out of it and the balloon text saying "HEY!" and water splashing out of it.... they get it, it's funny!
Again, text, "Did your Gravy Touch your peas?" ... but then there is this fantasticly simple but hiarious illustration of all these little peas with eyes and this horrible gravy pouring toward them and they are screaming run for your lives and there's the perfect line, "Gravy + Peas = Poison" in brackets... well that just sums up the way a child see's the whole issue of food touching food!
Do you have to know how to read to enjoy this book? NO and I think that is what makes a good picture book. When both the text and the illustration could stand alone and tell a story but when combined it's larger then either one of the two.