Minimum WordPress Plugin that adds <image> Tags to the RSS2 Feed. Add this file to a folder calles /wp-content/plugins/rss2-feed-with-images/

rss2-feed-with-images.php

<?php
/*
Plugin Name: RSS2 Feed with Images
Plugin URI: 
Description: Add image objects to the RSS2 Feed
Version: 1.1
Author: Jan Thiel
Author URI: https://hive-it.de
License: GPL2
*/

add_action( 'rss2_item', 'add_images_to_rss2_feed' );

function add_images_to_rss2_feed() {
	global $post;
	if ( has_post_thumbnail( $post->ID ) ) {
		$thumbnail = get_the_post_thumbnail_url( $post->ID, 'full' );
		echo("<image>{$thumbnail}</image>");
	}
}