Bootstrap 3 thumbnails: properly frame image

Bootstrap 3 thumbnails: properly frame image

In Bootstrap 3 i want design a row with 2 thumbnail so, following the
documentation, i have code:
<div class="container">
<div class="row">
<div class="col-md-3">
<div class = "thumbnail">
<img src="link" alt="150x150" style="width: 150px; height: 150px;">
<div class="caption">
<h3>Thumbnail label</h3>
</div>
</div>
</div>
<div class="col-md-3">
<div class = "thumbnail">
<img src="link" alt="150x150" style="width: 150px; height: 150px;">
<div class="caption">
<h3>Thumbnail label</h3>
</div>
</div>
</div>
What i obtain is the html page (i post an image excerpt) below:
How can i obtaine a frame like the one in the bootstrap documentation
(that properly enclose the images)?:

Note: my image dimension are 150px x 150px