Apple Photos Won't Import Any Image File

Where is your library located? If on an external drive how is the drive formatted and connected to your iMac?

Try launching your library with the Option+Command keys held down to repair the library. Then test importing again.

If that fails as a test launch Photos with the Option key held down and create a new, test library.  Import some photos and test to see if the same problem persists. Does it? This tells us if the problem is limited to your current library or is more wide spread. If the problem goes away then test again with your original library.

If it still won't import create and log into a new, basic, admin user account, Set up users, guests, and groups on Mac,  and see if the problem persists. This tells us if the problem is limited to your user account or is system wide.

Note:  do not launch any 3rd party apps in the account so as to not install any of the supporting files that most create.

If the problem isn't present then your original account has software installed that is causing the problem.  

That's a perfect example of Occam's razor. It's the simplest solution that works. Let's not 🔭 a 🎁 🐴 in the 😮.