By default, Magento will check the 'Exclude' box for you on all imported images, making them not show up as a thumbnail under the main product image on the product view.
# Mass Include UPDATE`catalog_product_entity_media_gallery_value` SET `disabled` = '0' WHERE `disabled` = '1'; # Mass Exclude UPDATE`catalog_product_entity_media_gallery_value` SET `disabled` = '1' WHERE `disabled` = '0';