/**

 * Add the post thumbnail, if available, before the content in feeds.

 *

 * @param string $content The post content.

 *

 * @return string

 */

function wpcode_snippet_rss_post_thumbnail( $content ) {

	global $post;

	if ( has_post_thumbnail( $post->ID ) ) {

		$content = '<p>' . get_the_post_thumbnail( $post->ID ) . '</p>' . $content;

	}

	return $content;

}

add_filter( 'the_excerpt_rss', 'wpcode_snippet_rss_post_thumbnail' );

add_filter( 'the_content_feed', 'wpcode_snippet_rss_post_thumbnail' );