div.image-hover img {
    -webkit-filter: grayscale(100%); /* Safari 6.0 - 9.0 */
    filter: grayscale(100%);
    transition: filter 0.3s ease-in-out;
}

div.image-hover:hover img {
    -webkit-filter: grayscale(0%); /* Safari 6.0 - 9.0 */
    filter: grayscale(0%);
}