When you say "stopped copying across to the project" from where are you copying? The media browser? The Photos app? What procedure are you using to transfer the photo to the project or timeline?
Sometimes when iMovie acts strangely, deleting/resetting preferences will cure the issue. To delete/reset preferences close and then 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 empty library. Reopen your old library to get back to your projects. Possibly you may need to reboot to setthe changes. Deleting preferences is a
safe procedure that will not cause data loss or disruption to your project.
You can also try creating a new project and see if the problem replicates there.
-- Rich