Where is Featured Image code stored in WP?

Well, I’m looking for source code of these two small containers (PHP backend, HTML frontend & JS used to attach the image):

I want to build a theme option based on Featured Image code, but I’m not able to find it. I’m pretty sure it is somewhere in wp-admin directory, but I’ve spent loong time there and found nothing.

Any clues?

Markup of that metabox is generated by _wp_post_thumbnail_html() function, contained in wp-admin/includes/post.php.