Event.observe(window,'load',init);

function init()
{
    
    $$('ul#mainNav img').each(function(elem,index)
    {
        Event.observe(elem,'mouseover',function(evt){
            Event.element(evt).src = Event.element(evt).src.replace("_off.","_on.");
        });

    	Event.observe(elem,'mouseout',function(evt){
    		Event.element(evt).src = Event.element(evt).src.replace("_on.","_off.");
    	});
    	
    });
}