Least annoying slideshow export format for iPhoto to iMovie
The H.264 codec that the QuickTime format uses is a part of MPEG-4.
To get the best results, tweak the options for MPEG4.

Go File → Export → Slideshow → Custom Export → Movie to MPEG-4 → Options….
Below is a screen cap of the options I recommend for you. You can adjust the resolution and bitrate depending on the quality you want (higher numbers = higher quality).