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.

lucia.intelisano

If post doesn’t have a set featured image, 
WP Autoset Featured Image Plus check if text contains an image. If yes, set this as featured image, 
if not, you can set any external url image. Otherwise, leave field empty. 
Easy and fast!
Features include:

WP Autoset Featured Image Plus

Check for an image inside text editor or use external images from anywhere as the featured image of your pages and posts.

Set All First Images As Featured vs WP Autoset Featured Image Plus