Were I to muster a defense of comics as potentially serious objects of rhetorical analysis in, say, a textbook, I would begin by pointing out that while there may not be a ready-made critical apparatus for comics as a genre, there exists a robust tradition of analyzing visual narrative. Consider Caravaggio's "Calling of St. Matthew":
I present the whole painting here so you can see what occupies the center of the painting. (Many reproductions of "The Calling" crop the upper third off. I will too to save bandwidth below.) If we assume that the eye of the viewer is drawn to the center, it becomes evident that Caravaggio didn't intend his static painting to be experienced as such. We light first on the boy's eyes and follow them:
To a Jesus finger. The Jesus finger points at the figure to the left of the boy:
This figure looks to Jesus to see whether he is the intended recipient of the Jesus finger and the viewer follows his gaze:
Now the viewer sees that while the Jesus gaze is less ambiguous than the Jesus finger, and that both are actually trained on Levi (soon to be Matthew):
Who attends to more mundane matters:
Compositionally speaking, there's much more to "The Calling" than this narrative—we could discuss how the shadow of the half-open door is complicit in framing the object of Jesus's attention, for example—but my point here is simply that by dint of an ambiguously crooked finger, Caravaggio encodes an active narrative into what could have been dead on the canvas.
That said, consider how much work is done by the title. Most Western art at the time represented familiar narratives, so outside the title no additional words are required to render the image comprehensible. But what happens if we secularize it? Given the 17th Century Italian attire of those around the table, it would not be unreasonable to submit that this painting could bear the title "Giovanni Points at the Man who Murdered His Father" or "Alessio is Stunned into Silence by the News that He Won the Lottery." Change the words that supplement the image and the meaning of the image changes. You no doubt see where I'm headed with this:
The interdependence of words and image is a relationship of long-standing. That the narrative potential of their interaction was not fully explored until the early 20th Century—and that that exploration would largely occur in a genre seemingly dedicated to aggressive juvenalia—seems to me more absurd than the contention that comics may be a respectable object of academic interest. However, the claim that the comics came into their own in the 20th Century doesn't mean that the word-picture relationship had not been previously explored . . . so tomorrow I'll talk about William Blake's "The Tyger."