I don’t think we will ever know for sure. But it was probably some Democrat operative.
Actually, they played with the brightness, and found the address of the kinkos that it was sent from, and tracked it to a particular guy that had a history of making anti-Bush stuff up. A few days later it was admitted. In amongst that was evidence that it wasn’t simply a gullible CBS, but that they lied about what their “handwriting analyst” said to them, and deceived him about what he was looking at.