var iValue, rNumber;

var image1 = new Image();
var image2 = new Image();
var image3 = new Image();
var image4 = new Image();
image1.src = "images/header_image1.jpg";
image2.src = "images/header_image2.jpg";
image3.src = "images/header_image3.jpg";
image4.src = "images/header_image4.jpg";

function DisplayImage() {
	iValue = Math.random() * 4;
	rNumber= Math.ceil(iValue);
	if(rNumber==0) rNumber++;
	document.headerimage.src = eval("image" + rNumber + ".src");
}