This plugin will set the first paragraph of the content as the excerpt and take the first image(if there are any) and set it as the ‘Image’ custom field(needed for some themes). When you hit the publish button, it will check to make sure that excerpt and Image custom field are filled. If not, it will ask you to confirm before publishing the post.
How to Use
Activate the plugin and update the Settings(Settings > Autofields) of the plugin to your liking. Then just write posts – Autofields will auto fill some fields for you.