The numShiftLights property defines the number of displayed shift lights (min: 0, max: 5).

The on-state shift light is taken from the given on-image. The off-state shift light is either taken from the background image at the given location, if the off-image is nothing, or taken from the given off-image. If the off-image is taken from the background image, the rectangle is completely replaced by the on-state image, if the state is on. So make sure, the surrounding pixels match.