Add thumbnail image of posts to WordPress RSS Feed -> Add to functions.php file of your theme

thumbnail-in-rss-feed.php

// 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
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, 'large');
    if (is_array($thumbnail)) {
      // Parse the URL and remove query parameters
      $parsed_url = parse_url($thumbnail[0]);
      $clean_url = $parsed_url['scheme'] . '://' . $parsed_url['host'] . $parsed_url['path'];
      echo '<media:content medium="image" url="' . $clean_url . '" width="' . $thumbnail[1] . '" height="' . $thumbnail[2] . '" />';
    }
  }
});