Edit in JSFiddle

img {
    -webkit-transform-style : preserve-3d;
    -webkit-transition-property : -webkit-transform;
    -webkit-transition-duration : .750s;
    -moz-transform-style : preserve-3d;
    -moz-transition-property : -webkit-transform;
    -moz-transition-duration : .750s;
    -o-transform-style : preserve-3d;
    -o-transition-property : -webkit-transform;
    -o-transition-duration : .750s;
    transform-style : preserve-3d;
    transition-property : -webkit-transform;
    transition-duration : .750s;
}
img:hover {
    -webkit-transform: rotateY(180deg);
    -moz-transform: rotateY(180deg);
    -o-transform: rotateY(180deg);
    transform: rotateY(180deg);
}