iMovie color management

I'm creating a variety of short movies featuring photography stills in iMovie, I need the transition and Ken Burn's cropping tools. I've tried a variety of import methods (tagged tiffs), including pre-converting all files to HDTV, sRGB, GenericRGB, etc, and iMovie totally hammers near blacks with loss of continuity and bad color crossovers. I've also tried various export methods, to no avail. Problems are most obvious with B&W imagery but close inspection of color reveals the same problems. Is it simply impossible to do this well with iMovie, or are there some techniques I need to use? I've googled about this a lot, seems to be a commom problem... Thanks, Tyler