var iValue, rNumber;

var img1 = new Image();
var img2 = new Image();
var img3 = new Image();
var img4 = new Image();
img1.src = "img/top_main1.jpg";
img2.src = "img/top_main2.jpg";
img3.src = "img/top_main3.jpg";
img4.src = "img/top_main4.jpg";

function displayImage(){
	iValue = Math.random() * 4;
	rNumber = Math.ceil(iValue);
	if(rNumber==0) rNumber++;
	document.image1.src=eval("img" + rNumber + ".src");
}