alaingg

Simple plugin that allows to include the images into your wordpress rss, rss2. 
The plugin use the post/page thumbnail or the firt attached image to the post/page gallery. viste the <a href="http://web-argument.com/2008/11/09/wp-rss-images-wordpress-plugin/" rel="nofollow ugc">plugin page</a> for details.

WP RSS Images

uwcals

Enables WordPress to include thumbnail and custom field data in its RSS2 feed.
How is works:
<ul>
<li>To retrieve post thumbnails: Add “post_thumbnail=1” to your feed’s link. (e.g. “http://www.mywpsite/feed/?post_thumb=1”)</li>
<li>To retrieve custom fields: Add “meta_keys=meta_name1[,meta_name2,meta_name3,…]” to your feed’s link (e.g. “http://www.mywpsite/feed/?meta_keys=post_quote,post_contributor” will retrieve the “post_quote” and “post_contributor” custom fields’ value, if these fields have been added to the post.)</li>
<li>To further customize your feed: use any other WP Feed options, as listed on https://codex.wordpress.org/WordPress_Feeds/.</li>
</ul>
Example: 
The http://news.cals.wisc.edu/feed/?cat=67&post_thumb=1&meta_keys=academic_info will retrieve all posts in category 67, along with their thumbnails (if available) and the value of the ‘academic_info’ field (if available).

UW CALS Extend RSS2 feed

Enables WordPress to include thumbnail and custom-field data in its RSS2 feed

WP RSS Images vs UW CALS Extend RSS2 feed