var visorURL = "imgs/visor1.jpg";

if (document.images) {
     var visor1 = new Image();
     visor1.src = "imgs/visor1.jpg";

     var visor2 = new Image();
     visor2.src = "imgs/visor2.jpg";
}

function changeVisor() {
     if (document.images) {
        if (visorURL == "imgs/visor1.jpg") visorURL = "imgs/visor2.jpg";
		
         document.myVisor.src = visorURL;
	}
}