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;
           }
        }