mbecher

If your theme supports the thumbnail feature introduced in WordPress 2.9, this plugin will display attached thumbnails on the admin area’s manage posts and manage pages screen, similar to the media management page.

Admin Thumbnails

If your theme supports Wordpress 2.9's thumbnail feature, this plugin displays thumbnails on the admin area's manage posts and manage pages  …

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.

Admin Thumbnails vs Thumbnail Cleaner