Edit in JSFiddle

img{
filter: url("data:image/svg+xml;utf8,<svg xmlns=\'http://www.w3.org/2000/svg\'><filter id=\'grayscale\'><feColorMatrix type=\'matrix\' values=\'0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0 0 0 1 0\'/></filter></svg>#grayscale");
    filter: gray;
    -webkit-filter: grayscale(100%);
}
img:hover {
    filter: none;
    -webkit-filter: grayscale(0%);
}

<img src="http://www.placekitten.com/400/300" />