<link href="//netdna.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css" rel="stylesheet" id="bootstrap-css">
<script src="//netdna.bootstrapcdn.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>
<script src="//code.jquery.com/jquery-1.11.1.min.js"></script>
<div class="row">
<div class="col-xs-12 col-md-6">
<a href="#" class="thumbnail">
<img src="http://placehold.it/350x150" alt="">
</a>
</div>
<div class="col-xs-12 col-md-6">
<a href="#" class="thumbnail">
<img src="http://placehold.it/350x150" alt="">
</a>
</div>
</div>
<div class="row">
<div class="col-xs-12 col-md-6">
<a href="#" class="thumbnail">
<img src="http://placehold.it/350x150" alt="">
</a>
</div>
<div class="col-xs-12 col-md-6">
<a href="#" class="thumbnail">
<img src="http://placehold.it/350x150" alt="">
</a>
</div>
</div>
