Include featured images in WordPress RSS feed post entries

functions.php

// add everything below this point to the bottom of your current theme's functions.php
function prepend_thumbnail_rss( $content ) {
  global $post;
  if ( has_post_thumbnail( $post->ID ) ) {
    $content = get_the_post_thumbnail( $post->ID, 'full' ) . $content;
  }
  return $content;
}
add_filter( 'the_excerpt_rss', 'prepend_thumbnail_rss' );
add_filter( 'the_content_feed', 'prepend_thumbnail_rss' );