importing pictures into the Photos app on a MacOS Big Sur MacBook Air

Try the following:

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. Then try importing into your original library again.

2 - next, if necessary, log into another vanilla admin user account, Set up users, guests, and groups on Mac,  create a test library and see if the problem persists. This tells us if the problem is limited to your user account or is system wide.

3 - if the problem goes away in the other user account go back to your user account and boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and test your primary library 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 the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

I have the same issue, all the time. I fix these errors by converting them to a newer format using Wondershare converter.

I have also since found Photos Sync - transfer app which also copies photos between my iphone and mac and supports live photos. So just as good as Photos app, but without these format errors.

How are you trying to import them, and where are you importing them from?

Can you open the files in preview?

Where is your library stored - on the system drive in the pictures folder, or somewhere else?

A vanilla user account is one that has no 3rd party add ons installed so there's no 3rd party software to possible affect performance, i.e. vanilla instead of rocky road. Sorry, bad pun?