#cf {
    position:relative;
}

#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 {
    position: relative;
}

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