Use a hidden album or locked folder in your phone’s Photos app
Move photos to a secure app with a passcode or biometric lock
Store photos in an encrypted cloud storage service
Put photos inside a password-protected archive file
Rename files to non-obvious names before storing them
Keep photos in a private folder with restricted access
Use a separate encrypted drive or USB device
Disable photo syncing on shared devices
Delete photos from shared galleries and backups if needed
Lock the app that contains the photos with a PIN, password, or fingerprint