//Dependancy: jQuery

jQuery(document).ready(function () {

    $("img.rollover").hover(

        function () {

            var source = $(this).attr("src").replace("_up", "_over")

            $(this).attr("src", source);

        }, 

        function () {

            var source = $(this).attr("src").replace("_over", "_up")

            $(this).attr("src", source);

        });

});