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)

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() ) : ?>
<?php the_post_thumbnail(); ?>
<?php endif; ?>


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