Edit in JSFiddle

.image {
    position:relative;
    width:400px;
    height:400px;
}
.image img {
    width:100%;
    vertical-align:top;
}
.image:after {
    content:'\A';
    position:absolute;
    width:100%; height:100%;
    top:0; left:0;
    background:rgba(0,0,0,0.6);
    opacity:0;
    transition: all 0.5s;
    -webkit-transition: all 0.5s;
}
.image:hover:after {
    opacity:1;
}