var timer;
var headers = ["1.jpg", "2.jpg", "3.jpg", "12.jpg", "5.jpg", "6.jpg", "7.jpg", "8.jpg", "9.jpg", "10.jpg", "11.jpg", "4.jpg"];
function starttimer() {
	timer = setInterval(headchange, 20000);
}
function headchange() {
	var randomNumber = Math.floor(Math.random() * 12);
	document.getElementById("header").style.backgroundImage = "url(http://yinnopiano.com/images/" + headers[randomNumber] + ")";
}