function initializeTopMenu() {
	
	$$('#topnav a').each(function(link) {
		var thisImage = link.down('img');
		thisImage.originalSrc = thisImage.src;
		thisImage.overSrc = thisImage.src.replace(/\.jpg/, "_on.jpg");
		
		(new Image()).src = thisImage.overSrc;

		link.observe('mouseover', function() {
			link.down('img').src = link.down('img').overSrc
		});
		link.observe('mouseout', function() {
			link.down('img').src = link.down('img').originalSrc
		});
			
		
		
	});
	
	
}

Event.observe(window, 'load', initializeTopMenu)