.imagen {
    height: 266px;
    position: relative;
    width: 440px;    
}
.imagen:before {
    border-radius: 4%;
	content: "";
	position: absolute;
	top: 0px;
	left: 0px;
	width: 440px;
    height: 266px;
	background: transparent;
    transition: background .2s linear;
}
.imagen:hover:before {
    background: rgba(0, 0, 0, .5);
 }