This WordPress Plugin does a very simple but useful task. It makes possible to add featured images to terms of any taxonomy, including custom taxonomies. As it name says, it’s also possible to use these term featured images as a fallback to the featured images in posts, when no image was selected. As an extra, you can select the priority at post type level, taxonomy level and term level.
As all of the plugins listed here, it also includes an option to leave your database 100% clean when uninstalling it.
You can download it at the WordPress official page.