It all boils down to:
1. Nuking the database
In your mariadb instance, execute
truncate catalog_product_entity_media_gallery_value;
truncate catalog_product_entity_media_gallery_value_to_entity;
delete from catalog_product_entity_media_gallery;
delete from catalog_product_entity_varchar where attribute_id in ( select
attribute_id from eav_attribute where attribute_code IN ( 'image', 'small_image', 'thumbnail', 'swatch_image', 'media_gallery' ) AND entity_type_id = 4 );
2. Nuking the media folder
In your Magento instance, execute
rm -rf pub/media/catalog/product
There, clean slate for you to populate.
