var iValue, rNumber;

var img1 = new Image();
var img2 = new Image();
var img3 = new Image();
img1 = "img/header_top01.jpg";
img2 = "img/header_top02.jpg";
img3 = "img/header_top03.jpg";

function display_image(){
iValue = Math.random() * 3;
rNumber = Math.ceil(iValue);
if(rNumber==0) rNumber++;
document.image_header.src=eval("img" + rNumber);
}