function preload_image(image_name, image_src) {
    if (document.images) {
        eval(image_name + ' = new Image()');
        eval(image_name + '.src = "' + image_src + '"');
    }
}
/* change_image() */
function change_image(image_name, image_src) {
    if (document.images) {
        document.images[image_name].src = image_src;
    }
}
/* preload */
preload_image('nav1','images/nav1_o.gif');
preload_image('nav2','images/nav2_o.gif');
preload_image('nav3','images/nav3_o.gif');
preload_image('nav4','images/nav4_o.gif');
preload_image('nav5','images/nav5_o.gif');