jewpi

Generates properly formatted post thumbnails on-the-fly for plugins and themes. 
Fallback thumbnails, remote images, Youtube videos supported.
<h4>Donations</h4>
If you like and use ThumbMaster you could <a href="https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=UV2FDM29SNY6W" rel="nofollow ugc">consider some donation</a> to the author.

ThumbMaster

Generates properly formatted post thumbnails on-the-fly for plugins and themes.

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.

ThumbMaster vs Thumbnail Cleaner