$(document).ready(function() {
    $('img.rollover').mouseover(function() {
        var img_src = $(this).attr('src');
        $(this).attr('src', img_src.replace("up-", "down-"));
    }).mouseout(function() {
        var img_src = $(this).attr('src');
        $(this).attr('src', img_src.replace("down-", "up-"));
    })
});