function initPage() {
    var _bg_div = document.getElementById('container2');
    if(_bg_div) {
	_bg_div.style.backgroundImage = "url(./images/bg-cont" + (Math.floor(Math.random()*4) + 1) + ".jpg)";
    }
}

if (window.addEventListener)
	window.addEventListener("load", initPage, false);
else if (window.attachEvent)
	window.attachEvent("onload", initPage);