Here is a hook to add the featured image to your rss feed on WordPress

functions.php

/*********** ADD IMAGES TO RSS FEED
you can set the position of the image changing the order like this 
$content = $content . get_the_post_thumbnail( $post->ID, 'thumbnail' );

*/
function featured_images_rss($content) {
  global $post;
	if ( has_post_thumbnail( $post->ID ) ){
		$content = get_the_post_thumbnail( $post->ID, 'thumbnail' ) . $content;
	}
	return $content;
}
add_filter('the_excerpt_rss', 'featured_images_rss', 1000, 1);
add_filter('the_content_feed', 'featured_images_rss', 1000, 1);