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。

ryanshoover

Cousett

Easily display a mobile featured image on your posts and page!
Up to 70% of website traffic today is on mobile devices. Sometimes WordPress does not quite resize the featured image in the best way. Avoid this and have full control over your featured images on any device with this plugin.
To add a mobile featured image, simply upload a new image in the meta box right below the WordPress Featured Image called Mobile Featured Image. The new image can be a specifically resized version of your featured image or an entirely new image targeted especially for mobile viewers.
You can add your mobile featured image without worrying about slowing down your site. The mobile image is added to the <code>srcset</code> attribute of your featured image. Vistors’ browsers will automatically download the best image for the size of their screen.
This plugin works by filtering the <code>wp_calculate_image_srcset</code> function and changing the url for all screens under 980w to the mobile featured image. 980w is a reliable breakpoint between the iPhone 6+ and the iPad, two common measures for when a mobile image would be needed.

GLS RSS Thumbnails vs Mobile Featured Image