The short answer
For a casual glance, lock the Hidden album (it is on by default since iOS 16) or put Face ID on the Photos app. For someone who knows your passcode, or for a file that must not be readable in a backup or on a computer, use a vault with its own passcode that encrypts on the phone.
Lock the Hidden album
It is locked with Face ID or Touch ID by default in iOS 16 and later. To check: Settings, Apps, Photos, Use Face ID. To remove the album from view entirely, turn off Show Hidden Album on the same screen. How to hide photos.
Lock the Photos app itself
In iOS 18 and later, touch and hold the Photos icon on the Home Screen and choose Require Face ID. Opening Photos then needs your face, or the passcode. You can also choose Hide, which moves the app to a locked folder in the App Library and removes it from the Home Screen.
Lock a note with photos in it
Notes can be locked with Face ID and the device passcode, or a separate note password. A locked note with a photo attached is a reasonable place for one or two images. It is not a gallery, it is not a vault, and by default it opens with the same passcode as the phone.
What all three have in common
- Face ID falls back to the phone passcode. Anyone who knows it gets in.
- The photos are still in the library or Notes, so they still sync to iCloud and still sit in backups.
- The files are not encrypted separately from the rest of the phone.
When you need a vault
If the person you are worried about knows the passcode, if the photo must not reach iCloud, or if you need the file to be unreadable on a computer or in a backup, lock it in a vault with its own passcode. Kryptis encrypts with AES-256-GCM on the phone, has its own passcode with Face ID optional and off by default, and adds a decoy vault and intruder alerts for the cases a lock cannot handle.
Choosing a vault
Look for a named algorithm, no account, and a privacy label with nothing linked to you. What to check.
