What version of iMovie are you using?
Assuming iMovie 10, open iMovie while holding down the Option and Command keys, and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. See if your photos display properly.
If no luck, try reducing the resolution of the photos. Put one on your desk top, open it in the Preview app on your Mac. In Preview, click on Tools/Adjust Size. In the box that opens reduce the long dimension number to a number less than 4000, say 2500. Then save and drag into iMovie. See if it displays properly.
Another thing you can try is putting the photos into a newly created project.
and see if they work there.
-- Rich