Louy Alakkad

This plugin will regenerate all missing image sizes for an image when it’s chosen as a featured image for any post. Very useful when you change your template. No settings whatsoever, just activate and forget.

Featured Image Resize

This plugin will regenerate all missing image sizes for an image when it's chosen as a featured image for any post. No settings whatsoever.

tamlyn

Unattach is a very simple plugin which allows detaching images and other media from WordPress posts, pages and other content types.
As of WordPress 4.2 this functionality is built in. For previous versions, the only way to detach a media item from a post is to delete either the post or the media item. This plugin solves that by adding an ‘Unattach’ link below each media file in the Media Library admin list view.
<a href="http://outlandish.com/blog/2011/03/unattach/" rel="nofollow ugc">Plugin by Outlandish</a>

Unattach

Allows detaching images and other media from posts, pages and other content types.

Featured Image Resize vs Unattach