I've been looking for this answer as well, although in my case it's how to set a platform or playlist to just inherit the image group setting from its parent (or global) so that if I want to reset all at the same time and then override it in specific platforms (like Box 3D for everything except Windows and DOS where I don't have a lot of 3D boxes).
So far I've checked in the platforms xml file and the games lists, but I can't find it anywhere.