In what direction is the photodiode placed in a CMOS image sensor?
Photodiodes can work either way. In the photovoltaic case they provide a forward bias voltage when illuminated. The other approach is to apply a reverse bias voltage and a current flows when illuminated. 

Looks like the 2nd circuit charges the gate capacitance and uses the PD to discharge it, and the 1st circuit discharges the gate capacitance and the PD charges it.