This is the only picture that looks like this, the other shots before or after look perfect, so I do not think it's dust or oil in the camera or on the sensor. The picture was taken during the summer months, so could it be an insect that flew into the frame at the right time? Leaning towards no, because it seems to be roughly square shape but oriented like a diamond.
The camera is a Canon DS126071 (EOS Canon Rebel XT 8MP DSLR)
ISO – 1600
Shooting – 1/640 sec. f / 16 22mm
Edit – let me be absolutely clear, I mean the giant black square on the subject's face, which was not intentionally put there to hide his identity before being posted.