images = new Array();

  images[0]  = "illustration_01.jpg";
  images[1]  = "illustration_02.jpg";
  images[2]  = "illustration_03.jpg";
  images[3]  = "illustration_04.jpg";
  images[4]  = "illustration_05.jpg";
  images[5]  = "illustration_06.jpg";

  index = Math.floor(Math.random() * images.length);
  document.write("<img src='images/"+images[index]+"' />");