ON THE FLY seems to be working. I haven't edited watermark.php. There is one problem: the watermark also appears on the thumb but just as large as on the supersize pic. I used a 380x80pix file by the way. I attached the thumb so you'll know what I mean.
Is there something I can do to either let the thumb(s) have a fitting watermark, or to not watermark the thumbs at all?