Magento 2 CLI tool for setting the first image on a product to the thumbnail. Offers the option to replace all thumbnail images or to leave those that are already set.
Install using composer
composer require creode/magento2-set-thumbnails
You don't need to run the maintenance or static content deploy commands if you are in development mode.
bin/magento creode:set-thumbnails
bin/magento maintenance:enable
bin/magento creode:set-thumbnails
bin/magento setup:static-content:deploy
bin/magento maintenance:disable