Real Thumbnail Generator allows you efficiently to regenerate your (existing) thumbnails in your media library. You can regenerate a single image or images in bulk. The plugin allows you to create a custom upload structure for thumbnails.

The regeneration of thumbnails can be up to 33% faster than other plugins due to chunked regeneration. Just install this plugin and it works with all your image and media files without any problems. Another helpful feature is, that unused image sizes can be detected and deleted, for example if you have switched the WordPress theme. It also supports WordPress Multisites.

Features
WordPress 5.0 ready
Only regenerate a set of image sizes, that means if you have registered a new image size you can only regenerate that one That means you can skip other image sizes.
Single regenerate images in media library table, grid and attachment view mode
Bulk regenerate in media library (table and grid mode)
Works great with PDF files (WP 4.7+ required)
Regenerate your whole media library
Works with your existing images
Custom upload structure for thumbnail pathes and filenames (you can define a pattern with placeholders)
Detect unused image sizes (for example after theme switch) and delete them
Can be up to 33 % faster than other plugins because RTG uses less requests to regenerate
Compatible with Revolution Slider
Compatible with touch devices
Supports multisite
6 months support Included
Forever free updates
GPDR / DSGVO: The plugin is collecting no personal data!