Yes all the icons are in the rco files, see the other thread I started about it, be best to talk there about this. the tool rcomage is attached there , it extracts the rco to images (if any), sounds(if any), main xml, and lang files (if any) , check the corresponding rco for your category, ie explore_category_psn.rco.
BTW: the main category icons cant be changed so easily, problem is that they are not taken from the xmb_normal.rco as you would expect, they are built into an sprx as far as i know. i suspect that the rco is just left over from old code.. from what i've seen sony were in the process of moving more and more resource files into the sprx files for extra security. Also the system category icons etc are not read from the sysconf rco like the other categories, probably to secure the otherOS block and things like that as the system settings category is so important.