There are many different screen aspect ratios and it's not possible to fit 428x240 image into any screen without distorting it. So there inevitably will be empty bars either at the sides or at the top/bottom.
You can choose letterbox mode and live with black bars. Or you can use Scale Outer mode and put some kind of nice looking border (sprites or tiled backgrounds) around your 428x240 area. You can also add a condition to to ignore mouse events if players click that border.