INSERT INTO images (id, width, height) VALUES ('gradient', 200, 200) RETURNING id;

WITH RECURSIVE
  width AS (SELECT width FROM images WHERE id = 'gradient'),
  height AS (SELECT height FROM images WHERE id = 'gradient'),
  horizontal(x) AS
    (SELECT * FROM width UNION ALL SELECT x - 1 FROM horizontal WHERE x > 1),
  vertical(y) AS
    (SELECT * FROM height UNION ALL SELECT y - 1 FROM vertical WHERE y > 1),
  _pixels(x, y, r, g, b) AS
    (SELECT x, y, mod(x, 255), mod(y, 255), 100 FROM vertical, horizontal)
INSERT INTO pixels (image_id, x, y, r, g, b) SELECT 'gradient', x, y, r, g, b FROM _pixels;