Anuar

Not all themes work the same way, and sometimes switching themes can lead to a lot of headaches. If you’ve previously used a theme that supports featured images and displays them on single post views, you probably want to ensure that these images are still shown if you switch to a theme that doesn’t show featured images here, without editing all of your old content.
This plugin automatically displays featured images at the top of the content area on single post views. It doesn’t check whether your theme displays them, so you’ll get double featured images if it does. It works via a filter, so if you ever switch themes again to one that does support featured images on single views, you can deactivate this plugin and things will go right back to normal. No information is saved in the database and no additional special image sizes are generated in this plugin.
<h4>Possible Core Material</h4>
This plugin may be material for WordPress core if it were to display featured images that exist when the current theme doesn’t support featured images. While this would reduce the potential for content loss, it would also have a significant affect on existing content.

Featured Image in Content

Not all themes work the same way, and sometimes switching themes can lead to a lot of headaches. If you've previously used a theme that supports  …

Adam Ainsworth

Does exactly what it says on the tin, if you want your Featured Image to be inserted before the content in your pages or posts, and your theme doesn’t support this, then this is the plugin for you.
No styling is done by default, this will be added in a future version.

Thumbnail Before Content

This is a simple plugin that automatically inserts the post thumbnail (featured image) before the post content.

Featured Image in Content vs Thumbnail Before Content