function addOnLoad (newOnLoad) {
    var oldOnLoad = window.onload;
    if(typeof oldOnLoad == 'function') {
        window.onload = function () {
            if(oldOnLoad)
                oldOnLoad()
            newOnLoad();
        }
    }
    else {
        window.onload = newOnLoad;
    }
}

addOnLoad(navRollover);

function navRollover() {
	var tbl = document.getElementById('headernav');
	var navs = tbl.getElementsByTagName("a");
	for(var i=0; i < navs.length; i++) {
		navs[i].imgNode = navs[i].childNodes[0];
		navs[i].origImg = navs[i].imgNode.src.substring(0, navs[i].imgNode.src.indexOf('.jpg'));
		var preLoadImg = new Image();
		preLoadImg.src = navs[i].origImg+'.jpg';
		navs[i].onmouseover  = function() {
			this.imgNode.src = this.origImg+'_ro.jpg';
		}

		navs[i].onmouseout = function() {
			this.imgNode.src = this.origImg+'.jpg';
		}
	}
}