Tobias Nyholm

Prevents an user to publish a post without a thumbnail. If a user presses the publish button without a thumbnail, the post will 
be saved as a draft. For more information see installation page or the 
<a href="http://www.webfish.se/wp/plugins/require-thumbnail" title="Webfish - WordPress plugin" rel="nofollow ugc">Webfish</a> homepage.

Require Thumbnail

Prevents an user to publish a post without a thumbnail.

tsaiid

Featured Image via URL is a plugin to generate the featured image of post from the image url specified. 
The image will be fetched back and saved into the Media Library with all supported sizes. 
If the featured image is already present, the plugin will do nothing.
This plugin is modified from Auto Post Thumbnail by adityamooley.
For more details, see http://tsai.it/project/wordpress-plugins/featured-image-via-url/

Featured Image via URL

Automatically generate the Featured Image from the specified image url in post.

Require Thumbnail vs Featured Image via URL