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

marynixie

This plugin generates Featured Image for all posts at once.
It is based on WordPres 2.9+ feature – Post Thumbnails. This plugin takes the first image (by default) from post body and assignes it as a post Featured Image. It helps if your theme supports post thumbnails feature and you want to assign Featured Image for all your existing posts.
Plugin performs mass manipulation on posts Featured Image property and uploads files if they are stored externally. Don’t forget to backup your files and database before using this plugin.
It displays log of success/fails. There is also a feature to remove all Featured Images from all posts – this only changes post setting, no actual files are removed.
Related Links:
<ul>
<li><a href="http://wordpress.shaldybina.com/plugins/generate-post-thumbnails/" title="Generate Post Thumbnails Plugin for WordPress" rel="nofollow ugc">Plugin Homepage</a></li>
</ul>

Generate Post Thumbnails

Tool for mass generation of Wordpress posts thumbnails using the post images.

Thumbnail Upscale vs Generate Post Thumbnails