Jess

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

Thumbnail Updater

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

Brianna Deleasa

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.
UPDATE: Now you can select images specifically for certain post types as well as taxonomy terms.

WP Random Post Thumbnails

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

Thumbnail Updater vs WP Random Post Thumbnails