Change default link of featured image

Hi,
I would like to have the featured image in my lighbox plugin. To do this, I need to override the featured image url.

The featured image by default links to the current page, like so:

<a href="https://www.example.com/this-page-url/" class="post-thumbnail">
<img src="https://www.example.com/wp-content/uploads/2016/09/imgage-name-default.jpg" class="featured-image">
</a>

For lightbox to pick up the image, it needs to link to the image itself. Something like this:

<a href="https://www.example.com/wp-content/uploads/2016/09/imgage-name-default.jpg" class="post-thumbnail">
<img src="https://www.example.com/wp-content/uploads/2016/09/imgage-name-default.jpg" class="featured-image">
</a>

Any ideas where I apply this override much appreciated.