Adding CSS to Featured Image for Attachment

Hello. Yesterday I found that I can link to attachment pages successfully with my featured images, however I'd like to add an overlay to the featured image and also images inside the article so that when a user hovers over it, they can see it is supposed to get larger.

I tried to implement the following script (The CSS3 version below the JS as it loads instantly)

http://webdesignandsuch.com/create-overlay-on-image-hover-jquery-css3/

It seems to be asking for an image though which I was unsure of how to define with my current featured image code:

<div id="post-thumbnail">

                    <?php if( has_post_thumbnail() ) : ?>
<a>">
    <?php the_post_thumbnail(); ?>
</a>
<?php endif; ?>

                    </div>

I'd like to apply this to images added inside the article as well if possible.

Ash