Reset all File Permissions to Default with Terminal?

Permissions on many of my image files seem to have gone awry. In this example, there are seven "everyone" entries, all with "Custom" privileges.

Neither Photoshop or Capture One will let me edit the files, and Finder won't delete the "everyone" permissions. Even when I'm logged-in as Admin, with the padlock unlocked, the minus sign is greyed out and Cmd + Backspace won't shift them either.

I can change each "everyone" to "Read and Write" but even when I do them all, it makes no difference, I still can't edit the files.

I understand it's possible to use Terminal to reset all file permissions to the default, so if anyone can describe how to do it, that would be great.

Of course, the system would be fully backed-up before I tried such a remedy.

Mojave 10.14.4 on an iMac Pro.

Thanks in advance, Andy