.myRollover, .myRollover:visited
{
background-image: url('templates/imagenes/apagada.jpg'); /* This would be the normal image */
}

.myRollover span {
text-indent: -5000px;
}

.myRollover:hover, .myRollover:active {
background-image: url('imagenes/encendida.jpg'); /* This would be the roll over image */
}
#menu a {
   ...
   background: url("templates/imagenes/apagada.jpg") top left no-repeat;
   }
#menu a:hover {
   ...
   background-image: url("imagenes/encendida.jpg");
   }
#menu a:active {
   ...
   background-image: url("imagenes/encendida.jpg");
   }