-- create the new image row with blank gallery_id:
  INSERT INTO images ... VALUES (...);
  -- atomically update the gallery_id field
  UPDATE images SET gallery_id = (SELECT 1+MAX(...) FROM images) WHERE ...;

##</code><code>##

  INSERT INTO images (gallery_id, title, description, fstop)
    SELECT 1+MAX(...), 'title', 'description', 'fstop' FROM images;