Open the Photos app
Select the photo you want to lock
Tap the Share button
Choose Hide
Confirm by tapping Hide Photo
Open the Settings app
Tap Photos
Turn on Use Face ID or Use Touch ID if available
Turn on Hidden Album
Open the Photos app
Go to Albums
Open Hidden
Use Face ID, Touch ID, or passcode to access it
For stronger privacy, move photos to a locked Notes note instead
Open Notes
Create a new note
Add the photo
Tap the More button
Choose Lock
Set a password if prompted