Derik has a short post that makes a nice note about how understanding of individual panels is sometimes conditioned by their context in a sequence.
I think this is a very important point that is well illustrated by his example. Sometimes, understanding of the elements in an individual panel relies on the information in other panels.
Most all cultures and individuals have little trouble decoding most propositional information in images (i.e. that an image of a horse means "a horse", or that an image of person is "a person", etc). However, certain individuals may have trouble comprehending the objects if their meaning is conditioned by a sequence. For example, this sort of meaning by context is often what children under four and other "non-visual language fluent" readers (or those fluent in a different type of system) struggle with.
Why is this important/interesting?
1) It lends validity to the idea that there is a fluency required for sequential image comprehension (and thus that there is a "system" guiding understanding to be fluent in).
2) It implies that even perceptual understanding (i.e. vision and object recognition) might rely on sequential understanding in these contexts, meaning that mere perception alone isn't enough to explain sequential image comprehension (i.e. again, a system for sequential images is necessary).
3) It hints that these sequential images were created to be in sequence and not just as random images strewn together. This is also a support against an image-to-image system of understanding like panel transitions, since transitions could function no matter what is thrown next to each other. This sort of execution has a more global scope: it's a whole sequence made to be a whole sequence, not just one after another.