Event.observe(window, 'load', function(event) {
    Event.observe($('bubble'), 'mouseover', function (event) {
        element = event.element();
        element.src = 'images/layout/contact_over.gif';
    });
    
    $('bubble').observe('mouseout', function (event) {
        element = event.element();
        element.src = 'images/layout/contact.gif';
    });
});