#cf {
  position:relative;
  height:420px;
  margin:0 auto;
}

#cf img {
  height: 100%;
  position:absolute;
  left:0;
  -webkit-transition: opacity 0.5s ease-in-out;
  -moz-transition: opacity 0.5s ease-in-out;
  -o-transition: opacity 0.5s ease-in-out;
  transition: opacity 0.5s ease-in-out;
}

#cf img.top:hover {
  opacity:0;
}