Thanks for those good ideas.
I see no one directly mentioned "disk images" in response to my query :
"And there is the choice of simply copying photos to CD or DVD or reducing the volume by forming .dmg disk images."
I did later try creating some disk images of folders, using 'compression', hoping to save space.
But the resultant disk images were little or no smaller than the originals, so that seems a waste of time, for JPG files, at any rate.
I will carry on just copying files in their original JPG format.