var nav = navigator
browserIE = ((nav.appName == "Microsoft Internet Explorer") && (parseInt(nav.appVersion) >= 4 ))
browserNS1 = ((nav.appName == "Netscape") && (parseInt(nav.appVersion) < 5 ) ) // Netscape < 4.7
browserNS2 = ((nav.appName == "Netscape") && (parseInt(nav.appVersion) >= 5 ) )// Netscape >= 6

// Precarga de imágenes ----------------------------------------
var i;
var imagenes = new Array("../images/portada/por1a.gif","../images/portada/por1b.jpg","../images/portada/por2a.gif","../images/portada/por2b.jpg","../images/portada/por3a.gif","../images/portada/por3b.jpg","../images/portada/por4a.gif","../images/portada/por4b.jpg","../images/portada/por5a.gif","../images/portada/por5b.jpg","../images/portada/por6a.gif","../images/portada/por6b.jpg","../images/portada/por7a.gif","../images/portada/por7b.jpg","../images/portada/por8a.gif","../images/portada/por8b.jpg","../images/portada/por9a.gif","../images/portada/por9b.jpg");
var lista_imagenes = new Array();
function cargarimagenes(){
for(i in imagenes){
lista_imagenes[i] = new Image();
lista_imagenes[i].src = imagenes[i];
}
}

function cambio(nombre,index){
                if ((browserIE)||(browserNS2)){

                        document.images[nombre].src = lista_imagenes[parseInt(index)].src;

                }
                else if (browserNS1){

                        document.layers["upc3"].document.images[nombre].src = lista_imagenes[parseInt(index)].src;
                }
}