function getObject(obj_name) {
	var obj;
	if (typeof obj_name == "string") {
		if (document.all) {
			obj = eval("document.all." + obj_name);
		} else {
			obj = document.getElementById(obj_name);
		}
	} else {
		obj = obj_name;
	}
	return (obj);
}

function switchBg() {
	var bgUrl = "url(images/topbg-" + Math.floor(Math.random() * 5) + ".jpg)";
	getObject("header").style.backgroundImage = bgUrl;
}