Download the files to a folder on the Desktop with Image Capture. See if you can open them with Preview. If so try the following as needed:
1 - 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)
2 - Log into another 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).
3 - boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and check there to see if the problem persists. Reboot normally and test again.
NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing.