ulfben

This plugin adds an “Use as Featured Image”-action to the Media Library. Very useful if you need to set the post thumbnail / featured image for a lot of posts.
<a href="https://wordpress.org/extend/plugins/bulk-select-featured-image/screenshots/" rel="ugc">A picture says more than a thousand words</a>.
//<a href="https://www.ulfbenjaminsson.com/" rel="nofollow ugc">Ulf Benjaminsson</a> 
Copyright (C) 2012 Ulf Benjaminsson (hello at my full name dot com).
This program is free software; you can redistribute it and/or modify 
it under the terms of the GNU General Public License as published by 
the Free Software Foundation; either version 2 of the License, or 
(at your option) any later version.
This program is distributed in the hope that it will be useful, 
but WITHOUT ANY WARRANTY; without even the implied warranty of 
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 
GNU General Public License for more details.
You should have received a copy of the GNU General Public License 
along with this program; if not, write to the Free Software 
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

Bulk-Select Featured Image

Allows you to select Featured Image / post thumbnail for your posts directly from the media library view.

Corey Salzano

This plugin does one thing, and that thing is to add your posts’ featured image to the RSS feed. It specifies the ‘large’ image size.
<h3>Open Graph Meta Tags are also Necessary</h3>
Many modern feed readers do not show the images added by this plugin. Feedly, for example, will not show images added by this plugin. Feed readers crawl article pages to find the image attached to posts in the open graph meta tag. Many SEO plugins add open graph tags for featured images. I like the (https://wordpress.org/plugins/opengraph/)[Open Graph] plugin, too.

Add Featured Image to RSS Feed

Adds the featured image attached to posts to the beginning of the post content and excerpt in RSS feeds.

Bulk-Select Featured Image vs Add Featured Image to RSS Feed