This is the only picture that looks like this, others taken sooner or later look good, so I do not think there's dust or oil in the camera or the sensor. The photo was taken during the summer months, could it be a mistake that flew into the frame at the right time? Leaning towards the no, since it seems to have an approximately square shape but oriented like a diamond.
The camera is Canon DS126071 (Canon EOS 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 mask his identity before publishing.