Hello Samash! Welcome to the C&C forum. I have to warn you - once you learn how to customize your gaming experience, it becomes a habit, then an addiction, then it takes over your life completely, ruining your health and relationships forever. So enjoy!
So you want to learn to index pcxs. Well, I could be boring and point you to the 'Tutorials' section of this forum, where you'll find lots of useful advice, but what fun would that be? Here, I might have something for you....
(rummages around in drawers, says, "damn!" , rummages around on shelves a bit more before muttering, "oh yes, here it is...")
A while back, just for poops and giggles, I made a little GIMP tutorial for making a new sort of improvement icon that Oni Ryuu had designed in response to Anthony Boscia's fix for the 'over-link bug' (that thread is
HERE)
The important part of it, for your purposes, is the 'indexing' portion, steps 9-16. See if this helps. It also shows my basic GIMP set-up, (note, for instance, that I always set the work-background to 'transparent') which I've found useful for most Civ projects:
Obviously, you should also check out the 'Tutorials' section of this forum, since this isn't an 'official' tutorial.