#cf {
  position:relative;
  margin:0auto;
  margin-bottom:3px;
  width:30px;
  height:30px;
}

#cf img {
  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;
}