Use Apple's Hidden album
In Photos, open the picture, use the More menu, and choose Hide. Find it later under Utilities → Hidden. Recent iOS versions can require Face ID, Touch ID, or your device passcode to open it. Menu names may vary by iOS version.
Understand what the Hidden album does
Hiding a photo removes it from the usual library views, but does not move it into an independent vault. Someone who can unlock your iPhone with the device passcode may also be able to access the Hidden album. Check Settings → Apps → Photos for relevant album visibility and authentication settings.
When a separate photo vault may help
For a different access boundary, a dedicated vault can import selected photos and require its own PIN. PrivateMoment stores vault media locally with encryption, supports an optional Face ID unlock, and does not require an account.
Check the original photo after import
Importing to another app does not automatically prove the photo is gone from Apple Photos, iCloud Photos, shared albums, recently deleted items, or existing backups. Verify copies and delete any originals you no longer want in those locations, according to your own backup needs.
Choose a setup you can recover
Before moving important photos, consider how you would recover them if the phone were lost. A local-only vault is not a cloud backup. Keep copies of irreplaceable files using a method you trust.
Note: iOS menus and behavior may change across versions. Confirm the options shown on your device.