if (document.images) {
   var inicio_up = new Image();
   inicio_up.src = "img/ini_up.png";
   var inicio_down = new Image();
   inicio_down.src = "img/ini_down.png";
   
   var hoja_up = new Image();
   hoja_up.src = "img/aten_up.png";
   var hoja_down = new Image();
   hoja_down.src = "img/aten_down.png";
   
   var donde_up = new Image();
   donde_up.src = "img/donde_up.png";
   var donde_down = new Image();
   donde_down.src = "img/donde_down.png";
   
   var cont_up = new Image();
   cont_up.src = "img/cont_up.png";
   var cont_down = new Image();
   cont_down.src = "img/cont_down.png";
}

function entra(boton) {
   if (document.images) {
      if (boton == 'inicio') {
         document.images[boton].src = inicio_down.src;
      }
      if (boton == 'hoja') {
         document.images[boton].src = hoja_down.src;
      }
      if (boton == 'donde') {
         document.images[boton].src = donde_down.src;
      }
      if (boton == 'cont') {
         document.images[boton].src = cont_down.src;
      }
   }
}

function sale(boton) {
   if (document.images) {
      if (boton == 'inicio') {
         document.images[boton].src = inicio_up.src;
      }
      if (boton == 'hoja') {
         document.images[boton].src = hoja_up.src;
      }
      if (boton == 'donde') {
         document.images[boton].src = donde_up.src;
      }
      if (boton == 'cont') {
         document.images[boton].src = cont_up.src;
      }   
   }
}