The pilot was giving scenic rides in the vintage airplane. The airplane was parked on the ramp, with the engine operating, and two passengers seated in the front seat. The pilot stated he set the parking brake, and exited the airplane to secure a baggage latch, when one of the passengers "bumped" the throttle. The passenger seated in the left front seat stated that he and his wife were escorted to, and seated in the airplane, while the pilot was talking with a passenger from the previous flight. He further stated that he was in the airplane for a few seconds, when he turned, and then heard the engine sound increase. The airplane accelerated across the ramp, and struck a parked airplane. Damage to the airplane consisted of bent left wing struts with the upper left wing bent downward approximately 30 degrees, and a bent left aileron. The passenger further stated that he did not receive any pre-boarding instructions or a pre-flight briefing. Examination of the airplane by a Federal Aviation Administration inspector did not reveal any preimpact mechanical malfunctions. It was noted that the engine controls were located on the left side wall of the cabin.