bootstrap centering thumbnail - html

Question

I'm taking the Free Code Camp course thing and the first project is to create a tribute page to whoever. Mine is on J Dilla, my favorite hip hop producer. God rest his soul. Anyways I'm trying to use a bootstrap thumbnail around a picture of him, with the text/caption also inside the thumbnail. My problem is that it messes up the centering and aligns the thumbnail to the left and I have no idea how to fix it. Here's the relevant code:
<style>
.cool-text {
font-family: Lobster;
font-size: 20px;
}
.image-centering {
display: block;
margin-left: auto;
margin-right: auto;
}
.vertical-centering {
margin-top: auto;
margin-bottom: auto;
}
.gray-background {
background-color: lightgray;
margin: 20px 100px 20px 100px;
border-radius: 5px;
}
.white-background {
background-color: white;
margin: 10px 560px 10px 10px;
}
</style>
<div class="gray-background">
<br>
<h1 class="cool-text text-center">J Dilla</h1>
<h2 class="text-center"><i>The one and only</i></h2>
<br>
<div class="span8 offset2">
<div class="img-thumbnail thumbnails">
<img class="image-centering" src="http://media.lessthan3.com/wp-content/uploads/2014/02/j-dilla-lessthan3.jpg" alt="The man himself."</img>
<p class="text-center">Dilla working on something ill, I presume</p>
</div>
</div>
<br>
</div>
</div>
Also if there's anything glaringly terrible about my code, I'd love some input on how to reformat it. This is my first time asking a question on stack overflow so forgive me if this is the wrong way to do so.