Lucy Tomas

Very simple plugin that will search for all the first images of your published posts, pages or custom post types and set them as the featured image.
<ul>
<li>You can choose the post type you want to run the plugin on.</li>
<li>Option to overwrite the already selected futured images.</li>
<li>Option to select From / To date.</li>
</ul>

Set All First Images As Featured

still works

It simply didn't recognize any images in any of the posts, and so didn't work. :(

Didn't see the images in the posts

IT works on 5.0.3

Sets the first image of your posts, pages or custom post types as the featured image.

toannth

Requires at least: 4.4 
Tested up to: 4.9 
Stable tag: 1.0.0 
License: GNU General Public License v2 or later 
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Resizes WordPress images on the fly using Nginx Image-Filter.

Ngx Image Resizer

Requires at least: 4.4 Tested up to: 4.9 Stable tag: 1.0.0 License: GNU General Public License v2 or later License URI: http://www.gnu.

Set All First Images As Featured vs Ngx Image Resizer