Wordpress - How to Add Image to Wordpress RSS-Feed with no Plug-in?

Here is a great example. How to display featured post thumbnails in WordPress feeds

paste this code snippet in your theme functions.php file

// display featured post thumbnails in WordPress feeds
function wcs_post_thumbnails_in_feeds( $content ) {
    global $post;
    if( has_post_thumbnail( $post->ID ) ) {
        $content = '<p>' . get_the_post_thumbnail( $post->ID ) . '</p>' . $content;
    }
    return $content;
}
add_filter( 'the_excerpt_rss', 'wcs_post_thumbnails_in_feeds' );
add_filter( 'the_content_feed', 'wcs_post_thumbnails_in_feeds' );

Based on the notes here and many other resources I read, I came up with this solution specifically to work with Mailchimp RSS to Email converter with the feed from Wordpress. Their templates use the <media:content> extension to the item element to populate their image macro. This code goes in the functions.php of the theme.

// Add namespace for media:image element used below
add_filter( 'rss2_ns', function(){
  echo 'xmlns:media="http://search.yahoo.com/mrss/"';
});

// insert the image object into the RSS item (see MB-191)
add_action('rss2_item', function(){
  global $post;
  if (has_post_thumbnail($post->ID)){
    $thumbnail_ID = get_post_thumbnail_id($post->ID);
    $thumbnail = wp_get_attachment_image_src($thumbnail_ID, 'medium');
    if (is_array($thumbnail)) {
      echo '<media:content medium="image" url="' . $thumbnail[0]
        . '" width="' . $thumbnail[1] . '" height="' . $thumbnail[2] . '" />';
    }
  }
});

The choice of image size 'medium' can also be 'thumbnail' if you want one smaller.

I tried the selected answer and got a really big image in my feed. I would recommend adding an image size to the code.

// display featured post thumbnails in RSS feeds
function WPGood_rss_thumbs( $content ) {
    global $post;
    if( has_post_thumbnail( $post->ID ) ) {
        $content = '<figure>' . get_the_post_thumbnail( $post->ID, 'thumbnail' ) . '</figure>' . $content;
    }
    return $content;
}
add_filter( 'the_excerpt_rss', 'WPGood_rss_thumbs' );
add_filter( 'the_content_feed', 'WPGood_rss_thumbs' );

I used 'thumbnail' for my feed, but 'medium' might work better for some sites.