Thinking in pictures stands nearer to unconscious processes than does thinking in words, and is unquestionably older than the latter both ontogenetically and phylogenetically.Monk is not making a wild claim here, but I think he's wrong all the same. Not that I can prove it, but it seems more likely that Wittgenstein was thinking of Freud's view that behavior, including things we say, reveals something about our inner life, or even that what is visible is a picture of what is invisible. Now that's a strange idea (a picture of the invisible), but Wittgenstein did say that the body is the best picture of the soul, and souls aren't visible. I associate this way of thinking with Schopenhauer, but Freud might have picked it up from him. It's also part of ordinary language, as when we speak about the expression on someone's face. We treat smiles and frowns as not just evidence of inner states but as manifestations of those states, the happiness or whatever pushing out through the face, visible in the face. And in a sense we get the very idea of an inner from this related idea that what we see is not all there is but is an outer, a phenomenon, the showing of something.
By the way, Monk refers to a composite picture of Wittgenstein and his sisters but that picture is not the one shown in the article. The picture Monk means is this: