S Hetjens

This plug-in adds the post thumbnail of articles to the RSS and Atom feed via Media RSS (MRSS) specification. Media RSS was 
invented by Yahoo as an extension to RSS feeds used for syndicating multimedia files (audio, video, image). This 
plug-in adds the post thumbnail defined by author of the article to RSS and Atom feeds of the blog. This plug-in 
activates the post thumbnail feature of WordPress, if the theme is not doing it. It is enough to activate the plug-in 
to work, it has no configuration parameters.
MediaRSS is a nice addition to your blog if you want to syndicate your content to other platforms. More and more 
platforms and RSS feed readers support it. The Notes App of Facebook recognize the post thumbnail and displays it in 
every newsfeed item.

Hetjens MediaRSS

This plug-in adds the post thumbnail of articles to the RSS and Atom-Feed via Media RSS specification.

wokamoto

This plugin modifies RSS feeds and ATOM feeds as you want.
<h4>How to use</h4>
Putting the following templates into the current theme folder, then you can use the customized feed templates instead of the default ones.
<ul>
<li>feed-atom.php – Atom Feed Template for displaying Atom Posts feed.</li>
<li>feed-atom-comments.php – Atom Feed Template for displaying Atom Comments feed.</li>
<li>feed-rdf.php – RSS 1 RDF Feed Template for displaying RSS 1 Posts feed.</li>
<li>feed-rss.php – RSS 0.92 Feed Template for displaying RSS 0.92 Posts feed.</li>
<li>feed-rss2.php – RSS2 Feed Template for displaying RSS2 Posts feed.</li>
<li>feed-rss2-comments.php – RSS2 Feed Template for displaying RSS2 Comments feed.</li>
</ul>
Note: 
When these feed templates are not in the current theme folder, WordPress’s default feed templates will be used.

Feed Template Customize

This plugin modifies RSS feeds and ATOM feeds as you want.

Hetjens MediaRSS vs Feed Template Customize