Centering .png image

[fkromer]

Centering an image (.jpg) in header.html works just fine:

<!-- Header -->
<header>
    <div class="container">
        <div class="row">
            <div class="col-lg-12">
                <img class="img-responsive" src="img/<image-name>.jpg" alt="">
              </div>
        </div>
    </div>
</header>

But why is an image (.png) in about.html left aligned?

<section class="success" id="about">
    <div class="container">
        <div class="row">
	<div class="col-lg-12 text-center">
               <img class="img-responsive text-center" src="img/skill_stack.png" alt="">
	</div>
        </div>
    </div>
</section>