I worked on a tiny bit of code for THREE hours, because there's little documentation on it. However, XML RSS 2.0 is still relevant.

So here's a working RSS 2.0 Example with Images:
http://chexed.com/feeds/MainNewsFeed.xml

I didn't make my XML feed available previously, because I normally run it through Google's FeedBurner, which marks it up real pretty.

FeedBurner URL:
http://feedproxy.google.com/ChexedcomMentalPleasures

The document which helped me is here:
http://stackoverflow.com/questions/3208873/images-on-the-description-of-an-rss-item ... This document was only found after finding many other suggestions which did not work, such as the "mrss media" tag, "xml image" tag.

It appears examples listed the w3c page are no longer active (404). The image tag it mentions can only be used for categories, not thumbnails or other images in your feed. It is still be useful, so here's the link to the main RSS 2.0 documentation on the web:
https://validator.w3.org/feed/docs/rss2.html

More documentation:
http://www.rssboard.org/rss-2-0-1#ltimagegtSubelementOfLtchannelgt