not all images are in the rco files indeed.

i know that the psn category's custom wallpapers, some icons (f@h, widgets, such things) reside in dev_flash/vsh/resource/explore/
also, some icons are nowhere to be found on current means.

a batch of rco files and otehr modules are hidden somewhere i believe is not accessible yet.
there's metldr which is a complete mystery, and i found references to non-existant rco/sprx files from the boot sequence along with a location called /dev_system...
whatever it is, can't really say. I lack time to properly research things at the moment.