Hmmmm. Possibly the problem is with QuickTimePlayer. When I tried a test with a pure-white still exported from iMovie as a video, it did seem a little more creamy white to my eye when played with QT player. Try playing the movie with the VLC player and Handbrake and see if you get a more pure-white display. VLC seemed to play it a little whiter.
You can try exporting from iMovie at the highest, less compressed, settings and see if that helps.
Also, you can try converting your movie to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures the issue.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and see if it plays with the pure white background that you want.
You might also try changing the format of the stills to TIFF, which is much less compressed than JPEG.
-- Rich
