well, that depends on you. in object editor you can set the icons for every unit/spell etc so if you want another so go in the object editor and find the entry for icon-game interface or something like that. then change it into the icon you want.
unused icons you can delete again in import manager by right-click. but make sure the icon you delete isnt used by any other !! or you may get an error - therefore: first change it then delete it.
this is for icons, but dont delete any skins !! they are always needed for the unit-model itself.