Custom RSS feeds in WordPress http://yoast.com/custom-rss-feeds-wordpress/ Additions per https://stackoverflow.com/questions/9123531/add-wordpress-featured-image-to-rss-feed/

custom-wp-rss-feed.php

<?php
/*
Template Name: Custom Feed
*/

$numposts = 5;

function yoast_rss_date( $timestamp = null ) {
  $timestamp = ($timestamp==null) ? time() : $timestamp;
  echo date(DATE_RSS, $timestamp);
}

function yoast_rss_text_limit($string, $length, $replacer = '...') { 
  $string = strip_tags($string);
  if(strlen($string) > $length) 
    return (preg_match('/^(.*)\W.*$/', substr($string, 0, $length+1), $matches) ? $matches[1] : substr($string, 0, $length)) . $replacer;   
  return $string; 
}

$posts = query_posts('showposts='.$numposts);

$lastpost = $numposts - 1;

header("Content-Type: application/rss+xml; charset=UTF-8");
echo '<?xml version="1.0"?>';
?><rss version="2.0">
<channel>
  <title>Yoast E-mail Update</title>
  <link>https://yoast.com/</link>
  <description>The latest blog posts from Yoast.com.</description>
  <language>en-us</language>
  <pubDate><?php yoast_rss_date( strtotime($ps[$lastpost]->post_date_gmt) ); ?></pubDate>
  <lastBuildDate><?php yoast_rss_date( strtotime($ps[$lastpost]->post_date_gmt) ); ?></lastBuildDate>
  <managingEditor>joost@yoast.com</managingEditor>
<?php foreach ($posts as $post) { ?>
  <item>
    <title><?php echo get_the_title($post->ID); ?></title>
    <link><?php echo get_permalink($post->ID); ?></link>
    <description><?php echo '<![CDATA['.yoast_rss_text_limit($post->post_content, 500).'<br/><br/>Keep on reading: <a href="'.get_permalink($post->ID).'">'.get_the_title($post->ID).'</a>'.']]>';  ?></description>
    <pubDate><?php yoast_rss_date( strtotime($post->post_date_gmt) ); ?></pubDate>
    <guid><?php echo get_permalink($post->ID); ?></guid>
  </item>
<?php } ?>
</channel>
</rss>

new_gist_file_1

// change <rss> to 
<rss version="2.0" xmlns:media="http://search.yahoo.com/mrss">

// add featued image as <media> 
<?php if(get_the_post_thumbnail()): ?>
    <media:content url="<?php echo wp_get_attachment_url(get_post_thumbnail_id($post->ID)); ?>" medium="image" />
<?php endif; ?>

// add specific size featued image as <media>
<?php if(get_the_post_thumbnail()): ?>
    <media:content url="<?php $image = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'imageSize'); echo $image[0]; ?>" medium="image" />
<?php endif; ?>