img = new Array();
img[0] = "/img/wrap_a.jpg";
img[1] = "/img/wrap_b.jpg";
img[2] = "/img/wrap_c.jpg";
n = Math.floor(Math.random()*img.length);
document.write("<style>");
document.write("#wrap {background-image: url("+img[n]+");}");
document.write("</style>");