function Randomize() {
	var images = new Array("Images/hd.jpg","Images/hmpg_img_1.jpg","Images/hmpg_img_2.jpg","Images/hmpg_img_4.jpg","Images/hmpg_img_5.jpg","Images/hmpg_img_6.jpg");
	var imageNum = Math.floor(Math.random() * images.length);
	document.getElementById("header").style.backgroundImage = "url('http://www.miamichamber.com/" + images[imageNum] + "')";
}
window.onload = Randomize;

<!--
function switchMenu(obj) {
	var el = document.getElementById(obj);
	if ( el.style.display != "none" ) {
		el.style.display = 'none';
	}
	else {
		el.style.display = '';
		
	}
}
//-->