<img src="images/12.jpg" class="img-rounded" width="250" height="200" alt="flower">
<img src="images/12.jpg" class="img-circle" width="250" height="200" alt="flower">
<img src="images/12.jpg" class="img-thumbnail" width="250" height="200" alt="flower">
just change the browser size and check that first image is responsive
<div class="col-md-6">
<img class="img-responsive" src="images/logo.png" alt="Logo" style="border:2px solid black">
</div>