You already have a private gallery, it’s called your library.
You can back it up the exact same way you (hopefully) back up your saves, since you should never reply solely on an outside entity to preserve your data. I keep 3 copies of my sims files when I backup: one elsewhere on my computer, one on a portable storage device and one in a Google drive reserved solely for this purpose. If there’s a system failure in one backup location, it’s not going to affect the others.
Just save the tray folder along with your saves folder and you can easily restore it if your computer dies or something.
That said, I would not mind the option of being able to mark certain pieces private or friends only so I could have a friend preview something before it goes out publicly on the gallery, but it should not be just a way of getting out of doing personal backups.
As i said in another reply, "Your Library" is not a private gallery, it's not linked to the internet or to the EA account, i recommend reading my others replys so that i don't need to write all that again.
