This is one of my favorite activities to do with the kids when it comes to teaching them about art. You can hear audible gasps when I show them a picture of the inside of the Sistine Chapel. I tell them they can paint anything they want to, however, they have to do it while lying on their backs! We tape paper to the underside of each table. They work with a partner who is there to assist with holding the paint upright for them (they thought it was hilarious when I actually went under one of the tables to demonstrate this :). We do use watercolors because I think there's less of a chance of it getting on their clothes, etc.
Maybe not like the Sistine Chapel, but they definitely had fun!