If you design your skin for a touch screen then you need only 2 images as there is no mouseover event. You need only the OFF and DOWN screen, even with the OFF and DOWN screen you have a mouseover effect as the button is highlighted with the mouse over it. This layout would be
RRScreen F-CNS "MENU"
Now you would have the mouseover event and the touch event on the screen.
Most all new skins are made now for transparency, so the user can select his/her owen background, therefore the images must be PNG format.