Require Thumbnail

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
Webfish homepage.

For more information see Webfish

1. Upload the plugin source to the /wp-content/plugins/ directory
2. Activate the plugin through the ‘Plugins’ menu in WordPress

The frequently asked questions is updated at the plugin’s homepage WordPress Require Thumbnail.