This information are stored in catalog_product_entity_varchar.

Before you get this information from database you must know attribute_id for base image attribute, you can get this id by:

SELECT `attribute_id` FROM eav_attribute where `attribute_code` ='image' AND entity_type_id = 4;
For me result is 85, now if we want get base image information for product with id 1020:

SELECT * FROM catalog_product_entity_varchar where `attribute_id`=85 and `entity_id`=1020
Hope it helps.