//Dependancy: jQuery
jQuery(document).ready(function () {
    $("img.rollover").hover(
        function () {
            var source = $(this).attr("src").replace("_off", "_on")
            $(this).attr("src", source);
        }, 
        function () {
            var source = $(this).attr("src").replace("_on", "_off")
            $(this).attr("src", source);
        });
});