WordPress - Genesis: Automatically set featured post images if missing

gistfile1.php

// Automatically set featured post images if missing
    function op_auto_feature_img() {
        global $post;
        $already_has_thumb = has_post_thumbnail($post->ID);
        if (!$already_has_thumb)  {
            $attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
            if ($attached_image) {
                foreach ($attached_image as $attachment_id => $attachment) {
                    set_post_thumbnail($post->ID, $attachment_id);
                }
            } else {
                set_post_thumbnail($post->ID, '3559'); // Default image
            }
        }
    }
    add_action('the_post', 'op_auto_feature_img');
    add_action('save_post', 'op_auto_feature_img');
    add_action('draft_to_publish', 'op_auto_feature_img');
    add_action('new_to_publish', 'op_auto_feature_img');
    add_action('pending_to_publish', 'op_auto_feature_img');
    add_action('future_to_publish', 'op_auto_feature_img');