Add Featured Images to WordPress Feeds

functions.php

<?php

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