Responsive Lightbox

HTML

<div class="overlay">
    <img src="http://placehold.it/2560x1440" alt="" />
</div>

CSS

html,body {
    margin:0;
    padding:0;
    width:100%;
    height:100%;
}

.overlay {
    position:fixed;
    top:0;
    left:0;
    background-color: rgba(0,0,0,.75);
    text-align:center;
    width:100%;
    height:100%;
    font-size:0;
}

img {
    top:50%;
    position:relative;
    transform: translateY(-50%);
    max-width:80%;
    max-height:80%;
}