var rules = {
	'ul.menu li' : function(element) {
		element.firstImage = element.getElementsByTagName('img')[0];
		element.imageSource = element.firstImage.src;
		
		element.onmouseover = function() {
			element.firstImage.src = element.imageSource.replace('.png', '-hover.png');
		}
		
		element.onmouseout = function() {
			element.firstImage.src = element.imageSource;
		}
	}
};
	
Behaviour.register(rules);