var imglist = new Array(
      "img/top_main.jpg",
      "img/top_main2.jpg",
      "img/top_main3.jpg",
      "img/top_main4.jpg" );
   var selectnum = Math.floor((Math.random() * 100)) % imglist.length;
   var output = "<img src=" + imglist[selectnum] + ">";
   document.write(output);