shprung

Less than 40 lines of php code so it will not slow your site and if all you 
need is to add an rss feed, give it a try.
You can even setup the output template – something many other plugin don’t 
offer.
<h3>A brief Markdown Example</h3>
<pre><code>short code examples:

[rss url="http://www.usacycling.org/rss/headlines.rss"]


[rss url="http://www.usacycling.org/rss/headlines.rss" no="20" target="_self"]

You can change the template used via wp-admin/options.php , look for
'rss_shortcode_template'
Template fields documentation:
 _TITLE_ : the rss feed title line
 _LINK_ : the link to the complete store, aka permalink
 _MSG_ : the short message/description; the first few 
 lines of the post/story
 _DATE_ : post date & time
 _TARGET_ : the link target
</code></pre>

RSS shortcode w/template

lets you embed rss feed in any WordPress content (e.g., pages, posts, custom post types) using Shortcode.

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.

RSS shortcode w/template vs RSS Image Feed