If you want to set Base , Small and Thumbnail image for all the added products please follow below steps
1- Open ‘eav_attribute’ table from phpmyadmin and get attribute value for IMAGE, SMALL_IMAGE and THUMBNAIL attribute ids are 87, 88, 89.
2- Run Below query in phpmyadmin
UPDATE catalog_product_entity_media_gallery AS mg,
catalog_product_entity_media_gallery_value AS mgv,
catalog_product_entity_varchar AS ev
SET ev.value = mg.value
WHERE mg.value_id = mgv.value_id
AND mgv.entity_id = ev.entity_id
AND ev.attribute_id IN (74, 75, 76)
AND mgv.position = 1;