You can regenerate the thumbnail images of the uploaded images with the help of the Lana Regenerate Thumbnails.

The plugin has two main functions:

Library Regenerate Thumbnails

You can regenerate thumbnails all of the images that’s already uploaded into the Media Library.

Media Image Regenerate Thumbnails

You can regenerate thumbnails more than one images uploaded into the Media Library.

Demo:
http://lana.solutions/lana-regenerate-thumbnails/wp-admin
Username: demo
Password: demo

Online Documentation:
http://lana.solutions/documentation/lana-regenerate-thumbnails/