Screengui getting a decal to change? (UNSOLVED)

It sounds simple right? Well I have tried time over time. I want to click on a screengui and when I click on it the decal changes to a light switch that is on and the light comes on. Than I want to click it again and it change to the off light switch. Anybody mind helping me write the script it should be very simple.

P.S. It has to be an ImageButton on the Screengui that changes decals.

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