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。

Pascal Birchler

required

Ulrich

Silvan Hagen

With this plugin, an MRSS namespace is added to the site’s RSS feed to include each post’s thumbnail.
WP Feed Post Thumbnail is very lightweight and only adds two small options under Settings -> Reading.
<h3>Contribute</h3>
If you would like to contribute to this plugin, report an issue or anything like that, please note that we develop this plugin on <a href="https://github.com/wearerequired/required-wp-feed-post-thumbnail" rel="nofollow ugc">GitHub</a>.
Developed by <a href="https://required.com/" title="Team of experienced web professionals from Switzerland & Germany" rel="nofollow ugc">required</a>

Feed Post Thumbnail

Tried refreshing permalinks, saving settings on the General settings page you've added the two checkboxes to, nothing makes a difference. No post thumbnail in the feeds...

Update! And then it started working!

I'm guessing this may have had something to do with either my hosting's caching or possibly even just my own browser cache. Works like a dream, thank you!

Not adding anything...

Adds MRSS namespace to the feed and uses post-thumbnail as media element in the feed. Settings available under Settings -> Reading.

GLS RSS Thumbnails vs Feed Post Thumbnail