All my photos are locked

Is the key in the upper right hand corner of the image? That's not a lock but a hard drive icon indicating it's a referenced image. Click on the icon and you can have Photos take to to the location on the drive where the source file is located:

It's not a lock. You're running a referenced library which is not recommended: Disadvantages of a Referenced Library in Photos. I suggest you select all of the photos in the library and use the File ➙ Consolidate menu option to convert the library to the recommend "managed" library.

That will work unless you've moved the source files from their original location during import.

Just wo we don't have to rotate our screens to view your screenshots in the future read the following:

How to take a screenshot on your Mac 

(⌘⬆︎3) Command-Shift-3         Capture the screen to a file

(⌘⬆︎⌃3) Command-Shift-Control-3 Capture the screen to the Clipboard

(⌘⬆︎4) Command-Shift-4         Capture a selection to a file

(⌘⬆︎⌃4) Command-Shift-Control-4 Capture a selection to the Clipboard

(⌘⬆︎5) Command-Shift-5         Invoke new screen grabber

You apparently were running, as some time, a reference library and have since moved the library or the source files to a different location. You'll have to find the source files and re-import them into the library.

Are you running a Managed or a Referenced Library?

A Managed Library, is the default setting, and Photos copies files into the Photos  Library when Importing. The files are then stored within the Library package

A Referenced Library is when Photos is NOT copying the files into the Photos Library when importing because you made a change at Photo -> Preferences -> General. (You unchecked the option to copy files into the Library on import) The files are then stored where ever you put them and not in the Library package. In this scenario you are responsible for the File Management.

Given that apple have a bad habit of calling applications unhelpful names - for instance Photos - the sentence

Photos on my old iMac are locked.

Is pretty ambiguous. So are these images in a Photos Library? Or in Folders in the Finder?

Does this involve Photos for Mac in some way? I ask as you're posting on this forum.