Product picture data.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
product_id	INTEGER		Product ID. (product.id)
index	INTEGER		Index of pictures. (0-origin)
filename	TEXT	NOT NULL	Relative file path from each product picture's folder including file extension.
mime	TEXT	NOT NULL	MIME type.
alt	TEXT		Alternation text.
disabled	INTEGER	NOT NULL	Whether the picture is disabled or not. 0: enabled 1: disabled

CREATE TABLE picture(
shop        TEXT,
product_id  INTEGER,
index       INTEGER,
filename    TEXT     NOT NULL,
mime        TEXT     NOT NULL,
alt         TEXT,
disabled    INTEGER  NOT NULL,
PRIMARY KEY(shop, product_id, index)
);