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

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

function galerie(nazevObrazku) {
if (ns3up || ie4up) {
imgOn = ("" + nazevObrazku);
document.hlavniObrazek.src = imgOn;
   }
}