eclev91

This plugin introduces a couple of nifty functions to replace <code>the_post_thumbnail()</code> & <code>get_the_post_thumbnail()</code> in exchange for the responsive <code>picture</code> element. Use:
<pre><code>get_the_post_picture(
 $fallback_image_size,
 array(
 $breakpoint_1=>$image_size_1,
 $breakpoint_2=>$image_size_2
 )
 [, $id, $atts]
);
</code></pre>
For example:
<pre><code><?php echo get_the_post_picture(
 'single-featured-sm',
 array(
 '(min-width:768px)'=>'single-featured-md',
 '(min-width:992px)'=>'single-featured-lg',
 '(min-width:1200px)'=>'single-featured'
 )
); ?>
</code></pre>
While this says 4.0 for minimum version, I would wager it’ll be fine on most older installations.

Picture Element Thumbnails

Get a picture element instead of an image tag for responsive images on your WordPress site.

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.

Picture Element Thumbnails vs Thumbnail Cleaner