Scott McCloud devotes a significant portion of his book to the discussion of what he prefers to call "icons," images that according to McCloud are used to "represent a person, place, thing, or idea" (McCloud 27). Starting with Magritte's famed image of a pipe, The Treachery 
of Images, McCloud goes on to explain that what icons are, and how they make up the vocabulary of comics in all their various levels of abstraction. What McCloud is talking about here, while he does not overtly use the term, is semiotics. In his book Signs in Contemporary Culture Arthur Berger explains that semiologists study signs, a sign being "something which stands to somebody for something in some respect or capacity" according to pioneering semiologist A. C. Pierce (Berger 1). Berger's book is much like McCloud's in that it seeks to find the underlying structure of communication. While McCloud is focused on the comic book, Berger looks at signs in general, "what they mean, how they generate meaning," and how we use them (Berger 1). Almost anything can be considered a sign, words, light, sound, or, in the case of the comic, images.
The study of signs covers an enormous amount of ground, and Berger touches on a number of aspects of semiotics, or he prefers to call it, semiology. He also uses Magritte's non-pipe, using it to illustrate the fact that signs can lie (Berger 64). An entire chapter of the book is spent studying the semiotics of cartoons, mostly of newspaper strips. Berger is using a different sort of semiotics than McCloud though. He's not interested in the mechanics of the comic book, as we are, but instead in the ideas behind cartoons that tell us things about the people reading and creating them. Where McCloud focuses on the simplicity of the smiley face, and our ability to identify with it, Berger examines the thinking behind Herriman's Krazy Kat, seeing in it the belief in "the triumph of illusion over reality" and "the victory of rebelliousness over authority" (Berger 54). He also relates comic books to historical artifacts such as the Bayeux Tapestry, and reinforces the idea that comics are perfectly valid for academic study (Berger 52, 58). Although Berger is on a different track than we are, he helps to open our eyes to a the broader world of signs and meanings is extremely valuable in attempts to relate comic books to other, more respected, forms of visual art.
In the early twentieth century, artists started to try some new things. The Cubists, Pablo Picasso being a leader in the movement, are of particular interest to us. Cubism, born in Paris and inspired by the abstracted simplicity of African art, threw the baggage of 
preceding Western art away, starting from what its founders saw to be the bottom up. They worked with the essential vocabulary of painting: form, space, color and technique, reinterpreting the use of these aspects of painting for their own personal expressive ends in an attempt to represent reality in a new way (Golding 17). While very abstract, especially for its time, Cubism was to its practitioners an art of realism. Cubist painters were to think in increasingly intellectual ways, trying not to "depict the world as the saw it, but as they knew it to be," choosing geometric planes as the proper means of representation (Golding 33). A Cubist would attempt to show more than one angle of an object at the same time using these planes, creating an image that, to the artist, would be even more real than a strict visual image (Golding 33). So, as we look here at Braque's 1911 painting of a man with a guitar, we may be visually confounded, but we are, in intention at least, seeing a more realistic image than one using more straightforward and traditional representation.
So, Cubism was an art movement that attempted to show something more real than just a plain two dimensional image. Cubists were trying to show the truth behind the image. It's not a great stretch to see the connection between this creative aspiration and semiotics: just as the cubists wanted to convey meaning beyond the straightforward visual, semiologists search for this meaning in signs, paintings included. When we read comics, we see more than just a cartoon of a person, ink on paper, we see a person, a character, much more than the sum of its parts. We'll get further into this a few paragraphs on, but first we can find even more to offer us in the art world.
Another art movement worth looking at is the Italian Futurism. The Futurists were similar to the Cubists in some ways, but much different in others. Futurism was less a style of art, and more of a motive for creating it (Taylor 17). Like Cubism, Futurism sought originality in art, aggressively rejecting the traditional, classical art that was especially overwhelming for artists in Italy (Taylor 9). In attempts to detach themselves from art that came before, Futurists focused on the "two forces that tend to destroy the concreteness of form: light and motion" (Taylor 30). These are aspects of reality that are difficult to capture in a visual format. especially. Our eyes capture motion as a series of images attached to each other, and in any given still instant, there is such a thing as movement, yet things move and we know it. Movement is the space between point A to point B, composed of infinite points. So, to try and capture this thing that exists largely in the human mind's inner perceptions, is also something which could be considered more realistic than what people can actually see. It's what we know. Let's see how Boccioni's 1911 painting The City Rises attempts to solve this problem:
As we can see, it's possible to portray motion in a painting through a number of means, directional strokes, rays of light, and dramatic poses being the method here. Still, the image hardly seems real. The meaning is clear though, so perhaps its more real than a traditional painting after all. Aside from the semiotic aspects of a futurist painting as an abstract representation of truth, this problem of motion is one that is very closely linked with the art of the comic book. Again, we will have to return to this later, as there is one last relation to the art world that we can discover.
Scott McCloud touches on another feature comics share with some of the more respected forms of visual art, that feature being line quality. Line quality is expressive, it is a way of encoding meaning, an action sure to delight semiologists, into something as simple as a single line. Here we see a page from Understanding Comics, in which McCloud displays examples and explains the meanings held within the different line qualities used by different comic book artists:
McCloud displays line quality in action.
It seems that McCloud is quite the semiologist himself. He's done precisely what Berger likes to do, finding the meaning behind the physical sign, and on one of the most minute levels possible. We can see here that the comic book represents more than the surface says, but this is only the beginning of our examination of what's going on underneath. Let's move on to what makes comic books really special, their representation of and interaction with that invisible fourth dimension: time.