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。

finnj

Frontier Set Featured will set featured image from images in the post if no featured image is set by the user.
<h4>Main Features</h4>
<ul>
<li>If no Featured Images is set on save, the plugin will:
<ul>
<li>Select the first image that is uploaded to the post, and set it as Featured Image</li>
<li>If still no Featured Image, select the first image from the post content, and set it as Featured Image</li>
</ul>
</li>
<li>The will work both from admin interface & frontend.</li>
</ul>
Note: Only images from the media library can be used as featured images (as per WordPress standard).
<h4>Frontier plugins</h4>
<ul>
<li><a href="https://wordpress.org/plugins/frontier-post/" rel="ugc">Frontier Post</a> – Complete frontend management of posts</li>
<li><a href="https://wordpress.org/plugins/frontier-query/" rel="ugc">Frontier Query</a> – Display lists and groupings of posts in post/pages and widgets.</li>
<li><a href="https://wordpress.org/plugins/frontier-buttons/" rel="ugc">Frontier Buttons</a> – Full control of tinymce toolbars and buttons buttons</li>
<li><a href="https://wordpress.org/plugins/frontier-set-featured/" rel="ugc">Frontier Set Featured </a> – Set featured image aut. based on post images </li>
<li><a href="https://wordpress.org/plugins/frontier-restrict-media/" rel="ugc">Frontier Restrict Media </a> – Restrict media access to users own media</li>
<li><a href="https://wordpress.org/plugins/frontier-restrict-backend/" rel="ugc">Frontier Restrict Backend </a> – Restrict access to the backend (wp-admin)</li>
</ul>

Frontier Set Featured

Frontier Set Featured will set featured image from images in the post if no featured image is set by the user.

GLS RSS Thumbnails vs Frontier Set Featured