Mark Jansen

<p>Get the first image from the content of a post and set that image as a featured image.</p>
<h4>Features</h4>
<ul>
<li>Select the (custom) post type for which you want to set the featured images</li>
<li>Ability to overwrite existing featured images.</li>
</ul>


WP Set Featured Image From Content

Get image from content to set as featured image

Jess

<p>Thumbnail Updater does exactly what it’s name implies: it updates thumbnails. WordPress 2.9 introduced support for<br />
post thumbnails but one problem is that the support is not backwards compatible—meaning that if you downloaded or created<br />
a theme that supports post thumbnails, any images uploaded prior to the activation of that theme will not have the new thumbnail size.</p>


Thumbnail Updater

A plugin for updating your thumbnails whenever a new thumbnail size is added with add_image_size()

WP Set Featured Image From Content vs Thumbnail Updater