gistfile1.php

add_filter( 'rss2_item', 'mailchimp_attached_images' );
 
function mailchimp_attached_images() {
    global $post;
    
    $thumbnail = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'large' );
    
    if ( $thumbnail ) {
        ?>
        <media:content url="<?php echo $thumbnail[0]; ?>" type="image/jpeg" medium="image" width="100%"></media:content>
        <?php
    }
}


add_filter( 'rss2_ns', 'flipboard_namespace' );
 
function flipboard_namespace() {
    echo 'xmlns:media="http://search.yahoo.com/mrss/"
    xmlns:georss="http://www.georss.org/georss"';
}