Ideas
MercAB3 I'm home now so I tried it - Yes it works! I also opened up the save file with Sims4Studio and can confirm I only saw one instance of the Save Game Custom Texture entry for that photo so that drives it home that the Copy action is just creating a new pointer to the same texture and because you're keeping the copy in a Sim's inventory and the game's currently broken saving routine does still scan all sims' inventories (even inactive ones too) it keeps it intact for both instances.
On the quality drop: I guess keep the lower quality copy in the sim's inventories and plan to dump them all later when the EA devs maybe some day actually fix this situation.
I also work in IT and I'd be fired if I let a bug like this linger this long, especially if I chose to sit on the fix instead of releasing it immediately, especially especially if I told my customers I had the fix but I was holding off on deploying it to wait until I had a chance to ask them for more money with a new product at the same time.
Bump1nTheNight Thank you so much! While I'll still have to do a restore at some point for some peripheral images like graduation photos (there's a mod for that) and paintings (not a big deal in my save file), being able to preserve my primary household's legacy is a huge win.