imgURL  = new Array();
imgURL[0]  = "http://www.terakichi.net/img/w01.jpg";
imgURL[1]  = "http://www.terakichi.net/img/w02.jpg";
imgURL[2]  = "http://www.terakichi.net/img/w03.jpg";
imgURL[3]  = "http://www.terakichi.net/img/w04.jpg";

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