Picture books are generally written and illustrated for young children who are not ready for chapter books, but they don't have to be. Most picture books are paired with a narrative story or simple concept words, though some are wordless as well.
The video below, created by author Katie Davis, has quotes from authors and illustrators answering the very important question "What Is a Picture Book?"
A to Zoo: subject access to children's picture books by 
Best Books for Children by 
The Pigeon Needs a Bath! by 
The Very Hungry Caterpillar by 
Goodnight Moon by 
Brown Bear, Brown Bear, What Do You See? by 
Corduroy by 
If You Give a Mouse a Cookie by 
Caps for Sale by 
Green Eggs and Ham by 
