div {
background-image: url('../img/pic.jpg');
background-position: center center;
}<div class="main-head">
<div class="main-head-over">
</div>
</div>.main-head {
position: relative;
width: 800px;
height: 600px;
}
.main-head-over {
position: absolute;
left: 0;
top: 0;
width: 800px;
height: 600px;
background-color: #000;
opacity: .4;
}