function navMOver () { this.src = this.src.replace(/off/,"on"); }
function navMOut () { this.src = this.src.replace(/on/,"off"); }

window.onload = function () {
  var nav = document.getElementById('nav');
  var navimages = getElementsByTagNames('img',nav);
  for(var i = 0; i < navimages.length; ++i) {
    navimages[i].onmouseover = navMOver;
    navimages[i].onmouseout = navMOut;
  }
}

////////////////////////////////////////////////////////////////////////////////
// Source: http://www.quirksmode.org/dom/getElementsByTagNames.html
function getElementsByTagNames(list,obj) {
	if (!obj) var obj = document;
	var tagNames = list.split(',');
	var resultArray = new Array();
	for (var i=0;i<tagNames.length;i++) {
		var tags = obj.getElementsByTagName(tagNames[i]);
		for (var j=0;j<tags.length;j++) {
			resultArray.push(tags[j]);
		}
	}
	var testNode = resultArray[0];
	if (!testNode) return [];
	if (testNode.sourceIndex) {
		resultArray.sort(function (a,b) {
				return a.sourceIndex - b.sourceIndex;
		});
	}
	else if (testNode.compareDocumentPosition) {
		resultArray.sort(function (a,b) {
				return 3 - (a.compareDocumentPosition(b) & 6);
		});
	}
	return resultArray;
}