Hi Nick - maybe the bug you refer to is indeed THE bug that affects pics that have already been edited using Ken Burns. So you know that re-editing under that circumstance creates a black image in photo preview because iMovie trashes the original imported pic. If the imovie trash is them emptied, the black screen results, and the only solution is to re-import the original photo. So, with iMovie 6, do not empty the trash!
This problem did not occur with iMovie HD (5), so if that is the 'HD' you are referring to, then you have some other problem.