Hi, Randy,
I've not seen the "missing plugin" error message with importing stills.
You didn't mention, but I assume that you are using latest version iMovie 10.1.9. If not, update to that version unless you have done so already.
Here are a few diagnostic procedures that you can try:
If still having the problem with iMovie 10.1.9 try opening iMovie while holding down the Option and Command keys and selecting to delete preferences from the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.
Create a new project, import your stills, and see if the problem still exists.
Open iMovie in a new library. To do that, hold down the Option key while opening iMovie and select to create a new library in the box that opens.
If still no luck, try reinstalling iMovie. Put your present version in the trash, but don't empty the trash, and then reinstall iMovie from the app store. After a successful reinstall, you can empty the trash of your older app.
You could also try reducing the resolution of the images that fail to import properly. From your desktop open them in the Preview app on your Mac, and then select Tools/Adjust Size, and reduce the long resolution number to below 4000. So, if for example your resolution it 4000 x 3250, reduce the 4000 number to, say, 2500. See it that makes any difference.
-- Rich