Can a picture have a purpose?

Some picture books are for adults, some are for children, and some are for both to share. Some picture books are serious, some are just fun, and some tell a story with a message, making them both. Some picture books are black and white; others are vividly bright. Some take their pictures from photographs; others try to seem real; and still others make no pretense, adding cartoon imagination to every scene. I kind of suspect picture books should be faster to read, but it's not always true, sin...