var imgSet = ["/images/banner/banner1.jpg", "/images/banner/banner2.jpg", "/images/banner/banner3.jpg", "/images/banner/banner4.jpg"];

function writeImg() {
	document.write('<IMG ID="bannerImg" ALT="ILRC Banner" SRC=' + imgSet[Math.floor(Math.random() * imgSet.length)] + ' />');
}