Decal script help?

Hey guys,

Please can someone give me a script that will change between 2 different images when a button is pressed.

Thanks.

Other button code:

BoolValue = script.Parent.Parent.EmergencyLight1.Toggle
function onClick()
    if BoolValue.Value == false then
        BoolValue.Value = true
        print(BoolValue.Value)
    else BoolValue.Value = false
    end
end

script.Parent.ClickDetector.MouseClick:connect(onClick)

PS: Had to choose a different tag cause there is no tag called "Decal"