document.observe('dom:loaded', function(e) {
    $$('.hoverimg').each(function (obj) {
        obj.observe('mouseover', function(ev) {
            obj.src = obj.src.substring(0, obj.src.length-4) + '_hover' + obj.src.substring(obj.src.length-4);
        });
    });
    
    $$('.hoverimg').each(function (obj) {
        obj.observe('mouseout', function(ev) {
            obj.src = obj.src.substring(0, obj.src.length-10) + obj.src.substring(obj.src.length-4);
        });
    });    
});