WordPress add featured image to RSS

zf-featured-image-in-rss.php

<?php
/**
 * Plugin Name: ZF Featured Image in RSS
 * Description: Adds the featured image URL (large) to its own RSS field for easy parsing with e.g. Feed Pull.
 * Author: Zeeland Family
 * Version: 1.0.0
 * Licence: GPLv3
 */

add_action( 'rss2_item', 'featured_image_field_in_rss' );
/**
 * Adds the featured image URL (large) to its own RSS field for easy parsing with e.g. Feed Pull.
 */
function zf_featured_image_field_in_rss() {
	global $post;

	if ( has_post_thumbnail( $post->ID ) ) {
		$thumbnail = wp_get_attachment_image_url( get_post_thumbnail_id( $post->ID ), 'large' );

		echo( '<image>' . esc_url( $thumbnail ) . '</image>' );
	}
}