<!-- Begin Preload Script -->

function newImage(arg) {
	if (document.images) {
		rslt = new Image();
		rslt.src = arg;
		return rslt;
	}
}

function changeImages() {
	if (document.images && (preloadFlag == true)) {
		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) {
		main_nav_01_over = newImage("images/buttons/btn_about_us_over.gif");
		main_nav_02_over = newImage("images/buttons/btn_market_sectors_over.gif");
		main_nav_03_over = newImage("images/buttons/btn_services_over.gif");
		main_nav_04_over = newImage("images/buttons/btn_quality_assurance_over.gif");
		main_nav_05_over = newImage("images/buttons/btn_careers_over.gif");
		main_nav_06_over = newImage("images/buttons/btn_contact_us_over.gif");
		preloadFlag = true;
	}
}

<!-- End Preload Script -->