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.

gilbarbara

Once activated and configured, this plugin will require users to fill selected fields before publishing posts, page, custom post types / custom fields.

Requird

Require users to fill selected fields before posting.

Post Requirements vs Requird