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

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

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