Hi there, the plugin does not add the thumbnail to the post content in your feed, but rather enhances the <item> element in your feed with something like:
<media:content url="https://blog.example.com/wp-content/uploads/2016/01/filename.jpg" type="image/jpeg" medium="image" width="800" height="490">
<media:title type="plain"><![CDATA[Title of your image]]></media:title>
<media:thumbnail url="https://blog.example.com/wp-content/uploads/2016/01/filename-170x170.jpg" width="170" height="170" />
<media:description type="plain"><![CDATA[Description of your image]]></media:description>
<media:copyright>Copyright on the image</media:copyright>
</media:content>
This follows the MRSS specification as mentioned here https://www.rssboard.org/media-rss#media-content
I hope this helps you implementing what you need.
