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

Brianna Deleasa

<p>This plugin allows you to upload a set of images via a settings page to be shown for posts without images. The images will be chosen at random on page load for any posts without a featured image (meaning each time the page is reloaded another random image will be chosen). Useful if your theme shows thumbnails for the posts, and you don’t want any posts without thumbnails.</p>
<p><strong>UPDATE:</strong> Now you can select images specifically for certain post types as well as taxonomy terms.</p>


WP Random Post Thumbnails

Allows you to select images to be shown at random for posts without a featured image.

WP Set Featured Image From Content vs WP Random Post Thumbnails