WP Random Post Thumbnails

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.