Import the icon and then change the path to what it is supposed to be (depending on what icon you have).
So basicly it is like this:
ReplaceableTextures\CommandButtons\BTNNAME of Icon.blp or .tga ReplaceableTextures\PassiveButtons\PASBTNNAME of Icon.blp or .tga ReplaceableTextures\CommandButtonsDisabled\DISBTNNAME of Icon.blp or .tga ReplaceableTextures\CommandButtonsDisabled\DISPASBTNNAME of Icon.blp or .tga