$(document).ready(function() {

$('img.rollover').hover(
    function() {
        //over
        var newImage = $(this).attr('src').replace(/^(.*?)(\.(?:gif|jpg|png))$/, "$1_over$2"); //adjust for your scheme
        $(this).attr('src', newImage);
    },
    function() {
        //out
        var newImage = $(this).attr('src').replace('_over', '');
        $(this).attr('src', newImage);
    }
);

});