img = new Array();
img[0] = "../images/under_a01.gif";
img[1] = "../images/under_a02.gif";
img[2] = "../images/under_a03.gif";
img[3] = "../images/under_a04.gif";
img[4] = "../images/under_a05.gif";

n = Math.floor(Math.random()*img.length);
document.write("<img src='"+img[n]+"' border='0'>");