I really love imovie but when trying to put different photos and audio together I had problems. Firstly I wanted to be able to have different videos. What was happening to me was that when I chose background music it would put this over every video. I figured out how to get around this. Have a different movie for each clip and then save each the camera roll. Then create a new movie and all your audio and sound tracks will woke indepently.
The other problem was imovie was cutting off heads and I couldn't rotate a clip. I exported the movie to camera roll, transferred to the pc with wifi app, and imported into a programme called wondershare. I could crop the video and still have heads of people and I could cut and rotate clips I had taken in portrait but moved the camera. It turned out great. Hope I help someone.