WTF
it appears the problem I'm experiencing may be unrelated to the icons.
my example:
I've put a barracks on the map, and made the footie, rifleman and knight all buildable from the start. I've given each of the three units one of the 3 custom icons.
The weird thing that happens is, only the footie unit's icon will show up, and the other 2 units get green boxes. I have tried interchanging the footie's icon between all of the 3 custom icons and they all work, but ONLY when given to the footie unit.
This is gonna take some serious investigating methinks.
oh yeah, the problem with those custom icons was that the doofus icon maker forgot to merge the alpha channels onto the image, thus creating the "black icon" that you would have probably seen in the game.