GLS Inc.

This plugin adds featured images (thumbnails) to the default WordPress RSS feed. 
WordPressのデフォルトのRSSフィードにアイキャッチ画像(サムネイル)を追加するプラグインです。
Once activated, featured images will be automatically added to the RSS feed. 
プラグインを有効化すると、自動的にRSSフィードにアイキャッチ画像が追加されます。
This plugin is provided as-is, without any guarantees of functionality or support. 
本プラグインは無償で提供されており、動作の保証やサポートはありません。
Standard RSS Feed URLs: 
– Post Feed: <code>[your-site]/feed/</code> 
– Category Feed: <code>[your-site]/category/category-name/feed/</code> 
– Custom Post Type Feed (example): <code>[your-site]/feed/?post_type=custom-post-type</code>

GLS RSS Thumbnails

Adds featured images to WordPress RSS feeds. / WordPressのRSSフィードにアイキャッチ画像を追加します。

Samuel Aguilera

NOTE: This plugin functionality is guaranteed by more than 10,000 active installs. But before installing this plugin bear in mind that its only purpose is to ADD the feature image ID to your post (in the same way that you would do using WP editor), it does not remove ANYTHING after deactivation. 
Before asking for support please read <a href="https://wordpress.org/plugins/easy-add-thumbnail/faq/" rel="ugc">FAQ</a> and <a href="https://wordpress.org/support/topic/please-read-before-posting-4" rel="ugc">this support thread</a>
<h4>How it works?</h4>
Checks if the post (any post type with thumbnail support, including pages) has already a featured image associated, and if not sets it using one of the following methods:
<ol>
<li>
Dynamically, for old published posts, the featured images are set only when needed to show them in the frontend. This means that the featured image is set (only first time) when a visitor loads the page where it needs to be shown.
</li>
<li>
For new content, the featured image is set in the publishing process.
</li>
</ol>
No options page to setup, simply install and activate.
If you want to exclude certain post type (e.g. pages), you can do it by using a filter. See <a href="https://wordpress.org/plugins/easy-add-thumbnail/faq/" rel="ugc">FAQ</a> for more details.
Easy Add Thumbnail doesn’t add any plugin data to your WordPress database. It simply adds the _thumbnail_id meta key with the ID of the attachment to be used as feature image (just the same that WordPress does when you set it manually).
Therefore it requires to have attached/uploaded at least one image to the post. If there’s not any image attached to the post, this plugin can’t help you.
Easy Add Thumbnail has not any control over how/when/where your featured image is displayed in your site, the display of the featured image in your site is TOTALLY controlled by your theme, this includes size, position, display it or not…
<h4>Requirements</h4>
<ul>
<li>WordPress 4.7 or higher.</li>
<li>PHP 5.6 or higher.</li>
</ul>

Easy Add Thumbnail

Automatically sets the featured image to the first image uploaded into the post (any post type with thumbnail support). So easy like that...

GLS RSS Thumbnails vs Easy Add Thumbnail