if (document.images) {  
            img1on = new Image();   img1on.src = "images/principala.gif"; 
            img2on = new Image();   img2on.src = "images/localizaciona.gif";  
            img3on = new Image();   img3on.src = "images/instalacionesa.gif";
            img4on = new Image();   img4on.src = "images/entornoa.gif";
            img5on = new Image();   img5on.src = "images/rutasa.gif"; 
            img6on = new Image();   img6on.src = "images/reservasa.gif";


            img1off = new Image();   img1off.src = "images/principal.gif"; 
            img2off = new Image();   img2off.src = "images/localizacion.gif";  
            img3off = new Image();   img3off.src = "images/instalaciones.gif";
            img4off = new Image();   img4off.src = "images/entorno.gif";
            img5off = new Image();   img5off.src = "images/rutas.gif"; 
            img6off = new Image();   img6off.src = "images/reservas.gif";

        }

    function imgOn(imgName) {
            if (document.images) {
                document[imgName].src = eval(imgName + "on.src");       
            }
    }

    function imgOff(imgName) {
            if (document.images) {
                document[imgName].src = eval(imgName + "off.src");        
            }
    }