"But perhaps the biggest obstacle to reviewing these works is that they are neither fish (totally text) nor fowl (totally art). There are very few of us who know how to review this genre. Even when we do address works like "Persepolis" and "Maus," it is the text that is usually examined most closely, with commentary on the artwork brought in as an afterthought. What should be considered, it seems to me, is the interplay between the two art forms, which lies at the heart of why these works differ from any other." (2004, 'Comic books for big people', http://www.poynter.org/profile/profile.asp?user=4399)
I've set myself the task of writing up book reviews of several relevant novels using literary terms to describe how the graphic elements are integrated into the narrative. Off the top of my head, I expect to find examples of chapter head illustrations used as foreshadowing (I have done this in some text designs recently), photographs/illustration as dramatic irony (the reader is shown something the protagonist isn't), photographs for flashback. Hopefully this will show that graphic elements can be described in the same terms as literary devices.