The pilot stated that the purpose of the flight was to make numerous takeoffs and transport skydivers to an adequate jumping altitude. The first takeoff was uneventful, and after the skydivers egressed the airplane, the pilot returned back to the airport. During the landing, the airplane bounced three times down the runway. The pilot taxied to the hangar and without shutting down the engine, boarded the second load of skydivers. Shortly thereafter, the pilot departed and during the initial climb, he attempted to retract the landing gear. The landing gear would not retract and the pilot decided to continue the flight with the landing gear extended. After the skydivers jumped, the pilot landed without incident. He taxied back to the hangar and shut down the engine. After exiting the airplane he noticed that the propeller tips were bent. As a result of the impact, the firewall was substantially damaged. The pilot reported no pre impact mechanical malfunctions or failures with the airplane that would have precluded normal operation.