zggz

If your theme makes thumbnail pictures links (which casuses a useless refresh on click), 
use this plugin to disable that. That’s it. Once enabled, it will make every featured image 
(on a post/page) unclickable.
<h3>Contributing</h3>
If you have any requests, bugs, or improvements, leave them at the GitHub for this plugin 
<a href="https://github.com/zggz/unclickable-featured-image" title="GitHub Repository" rel="nofollow ugc">GitHub repo link</a>

Unclickable Featured Image

A WordPress plugin that forces all featured images to NOT be links

thaikolja

Thumbnails Cleaner helps you keep your WordPress installation tidy by removing and regenerating thumbnails.
You can control all functions through the plugins’ main page found under “Tools”. You can:
<ul>
<li>Analyze your uploads directory, giving you an overview how many original files and thumbnails there are.</li>
<li>Clean thumbnails which means deleting them while keeping the original files.</li>
<li>Restore thumbnails to start out with a clean uploads directory. This function requires the plugin <a href="https://wordpress.org/plugins/regenerate-thumbnails/" rel="ugc">Regenerate Thumbnails</a>.</li>
<li>Create backups of your uploads directory to keep your files safe in the unlikely event that there is a problem.</li>
<li>Restore backups in case you have lost a file that was not supposed to be deleted.</li>
</ul>
Please see the <a href="https://www.koljanolte.com/wordpress/plugins/thumbnail-cleaner/documentation/" rel="nofollow ugc">official website</a> for a full 
documentation.

Thumbnail Cleaner

Cleans up your WordPress installation by removing and regenerating thumbnails.

Unclickable Featured Image vs Thumbnail Cleaner