Start with iPhone's built-in protections
Use a device passcode and Face ID or Touch ID, keep iOS updated, and review who knows your passcode. The Photos app's Hidden album can be locked using your device authentication.
Device passcode versus an app-specific PIN
Apple's Hidden album relies on device authentication. A separate app-specific PIN creates an additional access step, though it cannot protect a compromised or already-unlocked device in every scenario.
Use PrivateMoment as a separate vault
PrivateMoment offers a 6-digit PIN, optional Face ID, local encrypted storage, automatic locking, and a private camera that keeps new captures within the app instead of first adding them to Photos.
Don't mistake hiding for deletion
A photo may remain in Photos, Recently Deleted, synchronized libraries, exports, or backups after being copied. Inspect those locations if your goal is to remove the original.
Avoid overpromising privacy
No app can guarantee secrecy against every threat, including an attacker with broad device access. Decide what level of protection you actually need and preserve a recovery copy of important media.
Note: iOS menus and behavior may change across versions. Confirm the options shown on your device.