I've just read that "so important was [Sir John] Tenniel's work for the first of the Alice books (Alice's Adventures in Wonderland, 1865) that his protest at the poor printing of the first edition caused the book to be cancelled altogether."* This attests to Tenniel's understanding of the significance images have in affecting how text is perceived (not to mentioned how text can also alter the meanings in an image). Why are images often treated as trivial additions to an already 'complete text'? Why are picture books relegated only to young audiences?
In some ways, young readers are the few who can actually read images. As we learned how to read as children, text slowly took precedence over images until the pictures disappeared altogether. We continued to learn how to read verbally, but being able to read visually was dismissed (or just cut out of the budget) until we are left mostly visually illiterate. Because verbal language takes precedence in our education over visual language, a large part of the population will never learn how to read visually. One of the reasons the visual language was dismissed was because this is one type of literacy that can be learned on one's own, with no formal education, and this is why it's often assumed to be easy, simple, or needs an inherent talent. Not true - it still has to be learned, whether through formal education or through continued practice and exercise unaided.
Images that accompany text aren't just for children. It's a valid form of literacy that's been overlooked for too long.
*Jerome McGann, "The Socialization of Texts." The Book History Reader. New York: Routledge, 2001. p. 44