function rotateHeader(){
	
var headerList = new Array()
headerList[0] = "http://www.sas.upenn.edu/home/test/PPE/images/header_1.jpg";
headerList[1] = "http://www.sas.upenn.edu/home/test/PPE/images/header_2.jpg";
headerList[2] = "http://www.sas.upenn.edu/home/test/PPE/images/header_3.jpg";
headerList[3] = "http://www.sas.upenn.edu/home/test/PPE/images/header_5.jpg";

var headerImage = headerList[(Math.round(Math.random()*(headerList.length-1)))];
document.getElementById('topimage').style.backgroundImage = "url("+headerImage+")";


}