Fullscreen example below - simple bit of jQuery. I've used the same sized image for the thumbnail and large image for the example but to save on bandwidth reduce the size of the thumbnail images.
<!DOCTYPE>
<html>
<head>
<meta ="charset=UTF-8" />
<title>Full Screen Gallery</title>
<style>
html {
height: 100%;
}
body {
font-family: helvetica, sans-serif;
height: 100%;
margin: 100px 0;
}
.photo-overlay {
height: 100%;
width: 100%;
position: absolute;
top: 0;
left: 0;
background-color:#9F3;
background-size: cover;
}
.close-photo-overlay {
font-size: 30px;
position: absolute;
right: 50px;
top: 50px;
color: #fff;
z-index: 2;
padding: 6px 9px;
border: 1px solid #fff;
}
.imgBox {
width: 20%;
margin: 0 auto;
}
img {
max-width: 100%;
height: auto;
padding: 0 0 15px 0;
}
</style>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.4.0/css/font-awesome.min.css">
<script src="http://code.jquery.com/jquery-1.10.2.min.js"></script>
<script>
$(document).ready(function() {
$('.photo-overlay').hide();
$('.thumb').click(function(event) {
event.preventDefault();
var imageUrl = $(this).attr('href');
$('.photo-overlay').show().css('background-image', 'url(' + imageUrl + ')');
});
$('.close-photo-overlay').css('cursor' , 'pointer').click(function() {
$('.photo-overlay').hide();
});
});
</script>
</head>
<body>
<div class="photo-overlay">
<div class="close-photo-overlay"><i class="fa fa-times"></i>
</div>
<!-- end close-photo-overlay -->
</div>
<!-- end photo-overlay -->
<div class="imgBox">
<a href="https://upload.wikimedia.org/wikipedia/commons/6/63/Monarch_In_May.jpg" class="thumb"><img src="https://upload.wikimedia.org/wikipedia/commons/6/63/Monarch_In_May.jpg" /></a>
</div>
<!-- end imgBox -->
<div class="imgBox">
<a href="http://cdn.funcheap.com/wp-content/uploads/2011/06/Butterfly_Beauty.jpg" class="thumb"><img src="http://cdn.funcheap.com/wp-content/uploads/2011/06/Butterfly_Beauty.jpg" class="thumb"/></a>
</div>
<!-- end imgBox -->
</body>
</html>