When naming your icons, make sure you put the letters "BTN" at the start of the BTN icon filename. Put "DISBTN" at the start of the DISBTN icon filename. e.g. BTNAxe.tga , DISBTNAxe.tga
When importing your icons into your map, use the import manager, and:
For the BTN icon, set the import path to:
ReplaceableTextures\CommandButtons\
For the DISBTN icon, set the import path to:
ReplaceableTextures\CommandButtonsDisabled\