We demonstrate a 256 x 256 passive photon counting imager based on hybridization of back-illuminated silicon Geiger-mode avalanche photodiodes to an all-digital CMOS counting chip. Photon detection efficiencies in the 10%-20% are observed at visible wavelengths. The detection efficiency is currently limited by optical crosstalk that leads to elevation of dark count rates as the bias voltage on the photodiodes is increased. Both the time dependence of dark count activity during a gate time and the spatial structure of dark images were successfully explained using crosstalk-based models.