function diw_post_thumbnail_feeds($content) {
	global $post;
	if(has_post_thumbnail($post->ID)) {
		$content = '<div>' . get_the_post_thumbnail($post->ID) . '</div>' . $content;
	}
	return $content;
}
add_filter('the_excerpt_rss', 'diw_post_thumbnail_feeds');
add_filter('the_content_feed', 'diw_post_thumbnail_feeds');

add thumbnails to rss feeds

PHP