Jeremy Felt

In some cases, it’s necessary to provide a summary of each post in your primary RSS feed while still syndicating 
your content with full text to various parties. Dual RSS Feed Key allows you to enter a secret key to use with 
your RSS feed to display a full text version of a site’s feed to those who have the key.

Dual RSS Feed Key

Provide a "secret" full text RSS feed on WordPress sites with summary RSS feeds enabled.

tepelstreel

The RSS Image Feed doesn’t create a feed of images, but ataches the first image of the post to your feeds. Normally only the content has images showing. With our plugin there will be a clickable image even in the excerpt and in the content there will be only one image instead of all the images of the post. Since version 2.2 it works as a multisite plugin as well.
The plugin was tested up to WP 4.5 It should work with versions down to 2.7 but was never tested on those.

RSS Image Feed

The RSS Image Feed adds the first image of a post to your feeds, even in firefox and even if you only display the excerpt.

Dual RSS Feed Key vs RSS Image Feed