browserName = navigator.appName;
  browserVer = parseInt(navigator.appVersion);

  ns3up = (browserName == "Netscape" && browserVer >= 3);
  ie4up = (browserName.indexOf("Microsoft") >= 0 && browserVer >= 4);

  function showPix(imgName) {
  if (ns3up || ie4up) {
  imgOn = ("" + imgName);
  document.mainpic.src = imgOn;
   }
 }