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.

Stanislav Khromov

This plugin enables upscaling of thumbnails for small media attachments. The default behaviour of WordPress is to not upscale small images, which means 
you are never guaranteed a certain size for a thumbnail format. This plugin corrects that and upscales small images to the specified thumbnail dimensions. 
This is beneficial in grid layouts where images of different sizes can make the grid malfunction.
<a href="https://wordpress.org/plugins/thumbnail-upscale/screenshots/" rel="ugc">Check screenshots for an example!</a>
Important: Your thumbnails format must have the crop setting enabled to be processed by this plugin.
Usage
After you have enabled Thumbnail Upscale plugin, all your future uploaded images will be upscaled. 
For existing images, install and run <a href="https://wordpress.org/plugins/force-regenerate-thumbnails/" rel="ugc">Force Regenerate Thumbnails</a> 
This will upscale your existing media images!
<h3>Requirements</h3>
<ul>
<li>PHP 5.2 or higher strongly recommended</li>
</ul>
<h3>Translations</h3>
None. The plugin has no translateable strings.

Thumbnail Upscale

Whaaaaat? You don't use this plugin? It's good.

It’s still relevant god damn it

Enables upscaling of thumbnails for small media attachments

Picture Element Thumbnails vs Thumbnail Upscale