img {
-webkit-transition: -webkit-filter .3s ease-in;
-moz-transition: filter .3s ease-in;
-ms-transition: filter .3s ease-in;
-o-transition: filter .3s ease-in;
transition: filter .3s ease-in;
}
img:hover {
-webkit-filter: grayscale(100%);
-moz-filter: grayscale(100%);
-ms-filter: grayscale(100%);
-o-filter: grayscale(100%);
filter: gray;
}