Filtering post_thumbnail_html - wrap it with a link to image src so image plugins pay attention

add-link-to-featured-image.php

<?php 

function smith_add_link_to_post_thumbnail_html($html, $post_id, $post_thumbnail_id, $size, $attr) {
	$myurl = get_the_post_thumbnail_url($post_id, 'full');
	$openlink = '<a href="' . $myurl . '">';
	return $openlink . $html . '</a>';
}
add_filter('post_thumbnail_html', 'smith_add_link_to_post_thumbnail_html', 99, 5);