Add wrapper or change HTML output for post thumbnail in WordPress

There’s an awesome filter available called post_thumbnail_html if you want to add a quick wrapper or otherwise change the HTML output of your post thumbnails (which are printed with the_post_thumbnail() or get_the_post_thumbnail() in your theme).

Here’s an example where we add a surrounding div wrapper and also link the image to its permalink:

/** Add wrapper for post thumbnail **/
add_filter('post_thumbnail_html', function( $html, $post_id, $post_thumbnail_id, $size, $attr )
{
    ob_start();
    ?>
    <span class="featured-image">
        <a href="<?=get_the_permalink($post_id)?>">
            <?=$html?>
        </a>
    </span>
    <?php
    return ob_get_clean();
}, 10, 5);