by Aaron Hockley on WordPress.org
Featured Image Reminder will display a confirmation dialog box if you press the Publish button and the post doesn't have a featured image.

This is the message shown if you forget to include a featured image. Choosing "Cancel" gives you the opportunity to add your forgotten image. If you choose "OK" the post will be published.
Featured Image Reminder came about because some forgetful blogger (me) had a site where featured images were important and he kept publishing blog posts without the featured image being prepared.
When the plugin is installed and activated, and you try to publish a post without a featured image, a dialog box will appear asking if you really want to do that.