I was browsing through the DB, trying to find out where the media attributes of product-media-gallery-items are stored. For clarification: They are called "Roles" in the Admin-Frontend and "mediaAttributes" when interacting with them in Code.
There is the catalog_product_entity_media_gallery table that holds all pictures and then there is the catalog_product_entity_media_gallery_value table that holds information about the pictures, such as in which store-views they are placed and their position, but I wasn't able to figure out where those "Roles" are kept.