Trying to add attribute to my posts' featured image

post_thumbnail_html

To alter the html output, you need to hook the filter post_thumbnail_html

From Codex:

apply_filters( 'post_thumbnail_html', string $html, int $post_id, string $post_thumbnail_id, string|array $size, string $attr )

Filters the post thumbnail HTML.

This filter is called by get_the_post_thumbnailon line 177 of post_thumbnail_template.php after $html has been set with:

$html = wp_get_attachment_image( $post_thumbnail_id, $size, false, $attr );

Filter in context:

/**
* Filters the post thumbnail HTML.
     * @param string       $html              The post thumbnail HTML.
     * @param int          $post_id           The post ID.
     * @param string       $post_thumbnail_id The post thumbnail ID.
     * @param string|array $size              The post thumbnail size. Image size or array of width and height values (in that order). Default 'post-thumbnail'.
     * @param string       $attr              Query string of attributes.
*/
return

 apply_filters( 'post_thumbnail_html', $html, $post->ID, $post_thumbnail_id, $size, $attr );

Adding data-featherlight="mylightbox" example:

So adding your data-featherlight="mylightbox" would perhaps be something like:

add_filter('post_thumbnail_html', 'my_thumbnail_filter_method', 10, 5 );
function my_thumbnail_filter_method($html, $post->ID, $post_thumbnail_id, $size, $attr) {
     $id = get_post_thumbnail_id();
     $src = wp_get_attachment_image_src($id, $size); 
     $alt = get_the_title($id); /
     $class = $attr['class'];

    $html = '<img src="' . $src[0] . '" alt="' . $alt . '" class="' . $class . '" data-featherlight="mylightbox" />';
}

Two other action hooks are present on either side of $html being set within get_the_pos_thumbnail.

1. begin_fetch_post_thumbnail_html
2. end_fetch_post_thumbnail_html

but both are only passing $post->ID, $post_thumbnail_id, $size. So, it looks like filtering post_thumbnail_html is the best way to access the <img> html string.

A note on has_post_thumbnail(), since it seems to come up often: this method will return true if any images are attached to post, not specifically for the post thumbnail (aka featured image).

You can try like this:

if ( has_post_thumbnail() ) { // check if the post has a Post Thumbnail assigned to it.
        the_post_thumbnail( 'full', array( 'class'  => 'responsive-class' ) ); // show featured image
    }