How can I set height for Twitter bootstrap thumbnails?

How can I set height for Twitter bootstrap thumbnails?

I'm novice here :) I use twitter bootstrap (3.0) for my project and i'm
having trouble setting up thumbnail module.
The width is ok but i can't set up height of thumbnails (i can't align all
thumbnails at the same height).
How i can set one (standard) height for all the thumbnails and zoom/scale
image to center so they can fill all space without stretching the image?
Thank you!
<div class="row">
<!-- Thumbnail 0 -->
<div class="col-sm-6 col-md-3">
<a href="#" class="thumbnail">
<img src="img.png" alt="...">
</a>
<div class="caption">
<h4>Arctic MX-2, 8g</h4>
</div>
</div><!-- /thumbnail 0 -->
<!-- Thumbnail 1 -->
<div class="col-sm-6 col-md-3">
<a href="#" class="thumbnail">
<img src="blue.png" alt="...">
</a>
<div class="caption">
<h4>Arctic MX-2, 8g</h4>
</div>
</div><!-- /thumbnail 1 -->
</div><!-- /thumbnail -->