First we need to understand how images are stored in the database and what are the tables involved that stores the image name, entity_id, attribute_id and attribute_code.
catalog_product_entity_varchar is used for storing product images value.
catalog_product_entity_media_gallery_value_to_entity that joins catalog_product_entity_media_gallery and catalog_product_entity tables.
NOTE : PLEASE TAKE BACKUP BEFOREHAND
Step 1 : Delete all the product image attribute code values. Where 87, 88 and 89 are the attribute_id for attribute_code image, small_image and thumbnail respectively.
DELETE FROM catalog_product_entity_varchar WHERE attribute_id IN (87,88,89);
Step 2 : All the catalog product gallery images are stored in these three tables. They need to be removed from the database. Use your favourite database management application.
SET FOREIGN_KEY_CHECKS = 0;
TRUNCATE catalog_product_entity_media_gallery;
TRUNCATE catalog_product_entity_media_gallery_value;
TRUNCATE catalog_product_entity_media_gallery_value_to_entity;
SET FOREIGN_KEY_CHECKS = 1;
Step 3 : Log in to the Magento server as, or switch to, a user with permissions to write to the Magento file system.
cd <magento_root>/pub/media/catalog/product
rm -rf *
Type y and press Enter.