Well here is a plugin that will randomly pick an image that you upload through your back office media interface. You just have to make sure the file name begins with a [EW-] (for Use Feature) and it is unattached. Just upload it and don�t use it for a post.

Allow to publish post thumbnails on sidebar and on RSS.

Generate Featured Images automatically based on your post content (title, etc). Customize your featured image with the built in template editor.

With help of this plugin you can display the most popular posts on your blog.

Dynamically adds multiple featured image (post thumbnail) functionality to posts, pages and custom post types.

Facebook Album Post allows you to instantly create a Post with a Photo Gallery, a title and a Featured Image from a public Facebook Album.

Enforce certain post types to be published with a featured image with certain dimensions if specified.

Provides a widget that displays the featured image for a post or page, if a featured image has been set.

A simple plugin that force the download of images or pictures such as jpeg,png etc.

Post Thumbnail Slider is a light-weight, slick but powerful thumbnail slider making plugin that allows you to show the post thumbnail with the title in different styles and positions in a sliding mode.

Gabfire Media Module extends the functionality of Featured Image to support Videos and Default Post Images.

Simple way to displaying your recent posts sidebar, including thumbnails, category, and number options.

Set an image as the featured image at the same time as you insert it into the post content.

The ultimate widget for displaying posts, custom post types or sticky posts with an array of options.

Extended Featured Content Widget is a powerful, light weight wordpress plugin that helps you can organize your post/page content beautifully.

Add the featured image tag in your posts RSS feed. For standard RSS feed XML, there is no image tag definition. This plugin will show the post featured image url in tag. So other application will show an image with your post when they are using your website rss feed.

Display FAQs in an expandable concertina or accordion section. FAQs can be ordered and categorised, and their appearance can be customised.

Enable another featured image for all post types. Comes with a shortcode for displaying the extra featured image.

Enhance your WordPress REST API by adding a featured image URL field directly to API responses, improving performance by eliminating extra requests.

Post Filtration is a simple plugin which shows the total words of a post and thumbnail of the post at All Post section in dashboard. It has a great facilities that you will be able to filter your posts basis on Post Words Number and with and without the presence of Post Thumbnail.