Where does MacOS store old versions of Preview edited images?
Where are these stored?

There is a folder at the root level of the disk marked /.DocumentRevisions-V100, which stores the data for versioning of documents. This is owned by root and not in a user-friendly format, so is best left alone.

File formats that use Apple's CoreData APIs (e.g. Pages) create revisions using their own Undo list, so the data is preserved in the file as a series of undo instructions.

It's possible that some versioning is done on APFS volumes in a similar way to localsnapshots.

How can I delete them?


Select File > Revert To > Browse All Versions.
Once you're in the "Time Machine" interface, move the pointer to the top of the screen to recover the menubar.
Now, under File > Revert To, you will now have the option to Delete the selected version, or with ALT held down, Delete All Versions.