for the tactical display you need an lbm file, these can be edited using grafx, and possibly ultimate paint - i have not tested ultimate paint,
the lbm's are in \Medieval\Battle\UnitIcons

additional lbm files made by LK, LBA and myself are in the demo mods downlaod area,
unit icon packs http://www.totalwar.org/MTWFiles/Demo_Enhance.shtml

new bif files or folders for the \Medieval\Textures\Men\.. are not enabled at this point in time, this is why mix and match will work

hopefully the patch will allow.