Javascript light switch

I would like a very simple javascript that I can add to an existing web page that will display a picture. ? Basically, I'd like to have one picture on the left that is a "before" picture, then I'd like a graphic of a light switch that the user turns on to display an "after" picture on the right.? ? If the above description is unclear.? This is for a website for an electrician.? The idea is to have pictures of a remodeling project show a before and after picture.? The lightswitch is because it's a site for an electrician.? So, the picture on the left will show a before picture, the user hits the switch and the after picture will appear on the right.