The 291-hour private pilot reported that the single-engine airplane was parked on the ramp with the engine idling when a female taking pictures of the pilot and passengers inadvertently walked into the turning propeller. The pilot reported in the NTSB Form 6120.1/2 that after conducting a local area flight, he landed and taxied the airplane to the ramp. He parked the airplane parallel to the terminal building and kept the engine running. A female observer walked from the terminal building to the left, rear side of the airplane to photograph the passengers in the back seat. She then proceeded to the front of the aircraft where she was struck by the idling propeller. There was no damage to the airplane and the pilot and three passengers were not injured. The female that walked into the propeller sustained serious injuries. Night visual meteorological conditions prevailed at the time of the mishap.