There is a lot going on here, so bear with my 2 cents, keeping in mind I have not used imgkap much if at all. I think the idea Sean threw out is trying to provide a tool for many uses, one of them being a good way for OpenCPN to provide "layers" of various types of "user" information in an comprehensible and easily managed way.
Types of Information (not comprehensive)
-Weather overlays (weatherfax_pi)
data - (sorry Mike, almost forgot that)
-Grib? (maybe not)etc.
with the possibly of having capabilities such as these
* location (coordinates) included in the file format
* transparency & controls (maybe on off, maybe an adustment bar)
* saved views - remember which layers were on in a given view.
* order of rendering of layers, select which one last, remembering that.
Can a Georeferenced PNG be considered a layer? Can it be the tool needed?
Maybe baby steps are needed, but BCN has asked this question in a different way earlier, thinking about all the different types of data and uses and how it impacts O. It certainly does not hurt to think/discuss this carefully, so there is a reasonable plan for development with less backing up and undoing things. Perhaps even with some shared tools that can be used by several plugins, which will leverage capabilities (think of watchdog and ocpn_draw_pi developments using guid)