Automatically insert featured image on regular posts

With a child theme (download sample child theme here), locate this in single.php:

<?php get_template_part('inc/post-formats'); ?>

Replace with this: https://pastebin.com/uEcUN0ZM

Excellent, thank you Alex.

Great, thanks!

I combined it with a script found in the WordPress Codex to get the thumbnail to link to the large size image – very useful, as they are often aggressively cropped.