Photos in the cloud will not show up in the iMovie sidebar Photos item. They would need to be downloaded to your physical Photos library.
iMovie will recognize only photos that are in your system photo library, as designated in the the Photos app general preferences.
If the above check out O.K, try deleting preferences. To delete preferences, open iMovie while holding down the Option and Command keys and select 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. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
-- Rich