If images aren't showing up in any of the admin Media Gallery previews, then all you need to do is run the following commands:
php bin/magento media-gallery:sync php bin/magento media-content:sync
The media-gallery:sync command may time out at the command line. If it does, add the following line at the top of app/bootstrap.php.
php_value max_execution_time 3600
Increase this number until the command no longer times out.