<script language="JavaScript" type="text/Jscript">

sfHover = function() {
var sfEls = document.getElementById("nav").getElementsByTagName("LI");
for (var i=0; i<sfEls.length; i++) {
sfEls[i].onmouseover=function()
{
this.className+=" sfhover";
}
sfEls[i].onmouseout=function() {
this.className=this.className.replace(new RegExp(" sfhover\\b"), "");
} } }

if (window.attachEvent) window.attachEvent("onload", sfHover);

function newImage(arg) {
	if (document.images) {
		rslt = new Image();
		rslt.src = arg;
		return rslt;
	}
}

function changeImages() {
	if (document.images) {
		for (var i=0; i<changeImages.arguments.length; i+=2) {
			document[changeImages.arguments[i]].src = changeImages.arguments[i+1];
		}
	}
}

var preloadFlag = false;
function preloadImages() {
	if (document.images) {
		buttonoff_01_over = newImage("images/buttonoff_01-over.gif");
		navbutton_01_over = newImage("images/navbutton_01-over.gif");
		preloadFlag = true;
	}
}

</script>

changeImages('navbutton_01', 'images/navbutton_01.gif'); return true;

<a href="#"
	onclick="changeImages('navbutton_01', 'images/navbutton_01.gif'); return true;"> Mmm bacon</a>