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。

CK MacLeod

Category Featured Images Extended (CFIX) can ensure that posts, category archive pages, widgets and other elements of your site will always display a thumbnail or featured image* when called upon to do so by themes, templates, and plugins.
If a featured image has been individually set for a post, it will be used. If a featured/thumbnail image has not been set for a post, CFIX will first try to supply an image from one of the post’s categories or tags. It will use a Yoast SEO “Primary” Category’s image if available. If no category or tag image is found, the plugin will look for a parent category with an image. Finally, if no linked or related post, category, tag, or parent category image is found, a particular fallback tag or category image can be used if set as “global fallback.”
This plugin was initially based on <a href="https://wordpress.org/plugins/category-featured-images/" rel="ugc">“Category Featured Images”</a> (CFI) by Mattia Roccoberton. If you already have been using CFI, when you install this extended version, your already saved category image settings will be preserved and copied.
In CFI
<ul>
<li>If a post does not have its own featured image, a category featured image will be used if available.</li>
<li>If a post has more than one category with a featured image, the first available category image will be used.</li>
<li>If none of the post’s categories has a featured image, a parent category’s featured image will be used if available.</li>
</ul>
In CFIX
<ul>
<li>(New since 1.2) You can also set tag images. </li>
<li>You can designate categories or tags whose images will be avoided: This feature may be helpful when numerous posts use the same general category, and you wish to force the use of a more specific image.</li>
<li>You can also designate a category or tag whose image will be used as a last fallback when no regular featured image, category image, or parent category image is available. This feature may be helpful especially if your installation features many top-level categories, and you either do not wish to find images for each and every one, or you just haven’t done so yet. (One way to combine this feature and the previous one would be to designate the same category for both avoidance and last resort: The plug-in would always look for a more specific, not-to-be-avoided category, but fall back to the more general category otherwise.)</li>
<li>When a post has multiple category or tag images available, either the most recently added category or tag will be used. (This behavior is modifiable via hook or filter). A Yoast SEO Primary Category’s image will be used if available. </li>
</ul>
NOTE: IF UPGRADING FROM CATEGORY FEATURED IMAGES, DE-ACTIVATE IT TO AVOID CONFLICTS
As a further precaution, do not uninstall CFI completely until you are sure that CFIX is working as expected for you.
* The terms “thumbnail” and “featured” are used somewhat interchangeably in WordPress, even though many featured/thumbnail images will be displayed at much larger than thumbnail size.
<h3>Additional Info</h3>
<h4>Still to Come</h4>
<ol>
<li>Thumbnail and Featured Image Fallback: A plugin with all CFIX capabilities as well as options to use other images (from post content or as uploaded) for fallback purposes.</li>
<li>Supplementary Utilities: To handle thumbnail “false positives” (a problem for some imported or restored archives) or flush bad thumbnail data.</li>
</ol>
Check the <a href="http://ckmacleod.com/wordpress-plugins/category-featured-images-extended/" rel="nofollow ugc">CFIX home pages</a> for additional background, examples, documentation, and usage tips… or to <a href="http://ckmacleod.com/wordpress-plugins/category-featured-images-extended/get-support/" rel="nofollow ugc">contact the developer directly</a>.
<h4>Thanks!</h4>
All gratitude to Mattia Roccoberton for the basic code for his original Categorey Featured Images plug-in, which I had been using for years before I got around to extending it. Thanks to John Prusinski for recommending the Yoast SEO modification, encouraging me to implement it, and even throwing in a tip via Paypal! Thanks also to all of the developers and everyday code-hackers, far too numerous to name, upon whose work I have depended.

Category Featured Images Extended

Unable to install the plugin, getting this error : 
Fatal error: Can't use function return value in write context in /wp-content/plugins/category-featured-images-extended/category-featured-images-extended.php on line 730

Unable to install : Error

Set images for categories and tags, especially for fallback post thumbnails or featured images.

GLS RSS Thumbnails vs Category Featured Images Extended