var max      = 4;
  var nrImages = 4;
  function makeImages() {
    this[0]     = "images/topo/1.jpg";
    this[1]     = "images/topo/2.jpg";
    this[2]     = "images/topo/3.jpg";
	this[3]     = "images/topo/4.jpg";
    this.length = nrImages;
  }
  function makeLinks() {
    this[0]     = "index.html";
    this[1]     = "index.html";
    this[2]     = "index.html";
	this[3]     = "index.html";
    this.length = nrImages;

  }
  var vetImages = new makeImages();
  var vetLinks  = new makeLinks();
  var x = Math.round(Math.random()*max);
  var y    = max / nrImages;
  for(var cont = 1;cont*y <= max;cont++) {
    if (x <= (cont*y)) {
      document.write("<a href="+vetLinks[cont-1]+" target=_self><img  src="+vetImages[cont-1]+" border=0></a>");
      break;
    }
  }