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.
This plugin was built to download and set the featured image of a thumbnail for specific video source embeds. Currently supported video sources: YouTube, WordPress.tv
This plugin automatically sets featured image for your blog posts if post thumbnail is not set manually. This extracts keywords from your article title and content and searches for image in pixabay.com.