Separating Images in RSS FEed

Bear with me, this is a complicated question:

I’ve been trying to separate a featured image from my content in a custom RSS feed. The effect I”m trying to do is to comply with this MailChimp template: http://blog.mailchimp.com/rss-to-email-enhancement-for-publishers/
(it just means I can place the featured image independently of the text)

So … the example seems to show some kind of tag called <media:content….

What is that? Where does it go in my RSS feed? Everywhere I put it it seems to be either ignored or causes an error.