Can someone tell he how to add a hook in functions.php, so

Hello!

Can someone tell he how to add a hook in functions.php, so i can display the featured image right after the posts entry meta?

I have tried this code, but it does not work:

function post_img() {

if( is_singular('post')) {?>

<?php echo get_the_post_thumbnail($post_id, 'full'); ?>

<?php }

}
add_action('bp_after_entry_meta', 'post_img');

Thank you!