Moving the WordPress uploads folder to another location or uploading images via FTP/cPanel do not automatically create thumbnails for the images or results in existing thumbnails not being displayed. This can be frustrating when you create new articles and there are no previews for the images listed in your Media Library. Fret not as there is a plugin for that!
Regenerate Thumbnails is sufficient for the basic task. You can try to explore reGenerate Thumbnails Advanced for some additional features. Both plugins are compatible with WordPress version 5.2.2. Please note that we do not make any form of warranty whatsoever in writing about these plugins. Download and install them at your own risk.