show-post-thumbnail-in rss-feed.php

<?php 
/**
 * Show post thumbnails in rss feed.
 *
 * @param  mixed $content
 * @return void
 */
function wbcom_add_thumbnail_in_rss_feed( $content ) {
	global $post;
	if ( is_feed() ) {
		if ( has_post_thumbnail( $post->ID ) ) {
			$output  = get_the_post_thumbnail( $post->ID, 'medium', array( 'style' => 'float:right; margin:0 0 10px 10px;' ) );
			$content = $output . $content;
		}
	}
	return $content;
}
add_filter( 'the_content', 'wbcom_add_thumbnail_in_rss_feed' );