img.lazy {
    /* we set the opacity to 0 */
       opacity: 0;
}

img.lazy-loaded {
    /* prepare the future animation */
    -webkit-transition: opacity .5s linear 0.2s;
        -moz-transition: opacity .5s linear 0.2s;
                    transition: opacity .5s linear 0.2s;
    /* we set the opacity to 1 to do the magic */
    opacity: 1;
}