How can I lock photos on my Mac?

For this question, there are two useful methods:

One is using parental control, another method is locking photos with third-party apps.

You can read the article:5 Effective Photo Lockers That Helps Mac Users Encrypt Photos Easily.
https://www.cisdem.com/resource/photo-lockers-mac.html

It shows 5 available photo lockers for mac there. Choose the one you think is the best.

You can lock photos on your Mac by using the built-in feature called "FileVault". FileVault is a full-disk encryption feature that secures your Mac by encrypting the entire hard drive, including your photos.

To enable FileVault, follow these steps:

1. Click on the Apple menu in the top-left corner of your screen and select "System Preferences".
2. Click on "Security & Privacy".
3. Click on the "FileVault" tab.
4. Click on the padlock icon in the bottom-left corner of the window and enter your administrator password to unlock the settings.
5. Click on "Turn On FileVault" and follow the prompts to set up the encryption.

Once FileVault is enabled, all of your photos (and other files) will be encrypted and require your password to access. This will prevent unauthorized access to your photos even if someone gains access to your Mac.

If you want to further protect specific photos, you can also use third-party apps such as "Folder Lock" or "Secret Folder" to create password-protected folders or vaults where you can store your sensitive photos. These apps are available for download in the App Store or from their respective websites.