Can I add something like "rel=$post_id" to post thumbnail?

I need to add a rel attribute(which is equal to post id) to every post thumbnail image. It can be done by simple preg_replace, but that will reside in the function.php file. I can not think of a way to retrieve the subjected post id from there. However, I must need the post id in the attribute.
I actually want the output like this:
<img src="#something" rel="#i_need_the_post_id_here"/>
Any clue on this?