img:before {
content: "";
display: block;
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
background: rgba(0,255,255, 0.5); /* color you want, in this case, probably black */
-moz-transition: background .3s linear;
-webkit-transition: background .3s linear;
-o-transition: background .3s linear;
transition: background .3s linear;
}