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.

ryanpcmcquen

Makes local copies of all the linked images and pdfs in a post, adding them as gallery attachments.
Source & support:
https://github.com/ryanpcmcquen/import-external-attachments
<h4>Credits</h4>
This plugin is based on the work done in the “Import External Images” plugin by MartyThornley.
https://github.com/MartyThornley
HTTPS support added by IvanDoomer: 
https://github.com/IvanDoomer
PDF support added by bengreeley: 
https://github.com/bengreeley
Most of the JavaScript was rewritten from the original plugin, to reduce the 
number of global variables.

Import external attachments

Makes local copies of all the linked images and pdfs in a post, adding them as gallery attachments.

Featured Image Resize vs Import external attachments