iMovie expects to produce a movie in landscape mode, hence the problem.
You could try and select the part of the picture you want. Click on the image, press "C" on the keyboardand you get into the edit mode. You can then use the tools to crop the part of the image you want. It probably helps to choose 4:3 ration for your project.
The other alternative is again to use the edit mode, and use the "Ken Burns" effect. This allows you to pan the image, like a moving rostrum camera. You can specify start and finish points.