I recently discovered that the "Semiotics Institute Online" has a few full online courses that are offered related to comics. One of those is a class on metaphor pictorial theory taught by Charles Forceville, who has been looking a lot at comics and other media.
Of particular interest is lecture #7, which directly discusses conceptual metaphors in numerous comics, ranging from European comics to manga, Calvin and Hobbes to political cartoons. It's worth reading for a good primer on some of the more creative aspects of meaning-making in comics.
Forceville has also recently set up a blog about his research. The site has abstracts for many of his papers, and in some cases even has pdf downloads available. Again, of particular interest to readers of this blog may be his papers on comics, mostly concerning his work on metaphor theory as well as a categorization system of visual signs (that I admittedly have mixed feelings about).
Go forth and learn!