Figured this out. It's quite simple, but not explicitly stated in the docs. Just remove the media tag altogether and it will pull images from the post.
<a href="http://pinterest.com/pin/create/button/?url=<?php echo urlencode(get_permalink($post->ID)); ?>&description=<?php the_title(); ?>" class="pin-it-button" count-layout="vertical"><span><img width="29" height="29" src="<?php bloginfo('template_url'); ?>/img/socialButtons/share/icons/24/pinterest.png" alt="Twitter" /></span></a>