Chetan Satasiya

This plugin will add the restriction to publish post without featured image.

CS Require featured image to publish posts

This plugin will add the restriction to publish post without featured image.

carestad

If your site is depending on posts to have their thumbnail images set, this is the plugin for you. Also makes it possible 
to require tags in every new post.
When trying to save a post without at thumbnail or tags and when activated, the user will get a warning and the post will 
be saved as a draft instead.

Post Requirements

This plugin allows you to require posts to have thumbnail set and/or tags.

CS Require featured image to publish posts vs Post Requirements