Edit in JSFiddle

<div class="lightbox">
	<div class="lightbox__center">
		<img  src="http://placehold.it/600x400">
	</div>
</div>

.lightbox {
 position: fixed;
 display: -webkit-box;
 display: -ms-flexbox;
 display: flex;
 -webkit-box-pack: center;
 -ms-flex-pack: center;
 justify-content: center;
  -webkit-box-align: center;
  -ms-flex-align: center;
  align-items: center;
  width: 100%;
  height: 100%;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  z-index: 10000;
  background: rgba(57, 70, 78, 0.8);
}
.lightbox__center {
  width: 0;
  height: 0;
}
.lightbox img {
  position: absolute;
  max-width: 100%;
  max-height: 100%;
  -webkit-transform: translate(-50%, -50%);
  transform: translate(-50%, -50%);
  -moz-box-shadow: 0 12px 15px 0 rgba(0,0,0,0.25);
  -webkit-box-shadow: 0 12px 15px 0 rgba(0,0,0,0.25);
  box-shadow: 0 12px 15px 0 rgba(0,0,0,0.25);
}