functions.php

/**
 * Featured image in atom feed.
 * @method featuredtoRSS
 * @param  $string $content post content.
 * @return $string $content updated content w/image.
 */
function independence_featuredtorss( $content ) {
	global $post;
	if ( has_post_thumbnail( $post->ID ) ) {
		$content = '<div>' . get_the_post_thumbnail( $post->ID, 'medium', array(
			'style' => 'margin-bottom: 15px;',
		) ) . '</div>' . $content;
	}
	return $content;
}

add_filter( 'the_excerpt_rss', 'independence_featuredtorss' );
add_filter( 'the_content_feed', 'Independence_featuredtorss' );