You will also need to create your own PGP private key (if you don't already have one) for encryption/decryption and signature/verification.
OpenKeychain is an implementation of OpenPGP for Android (see LINKS) - available from the Google Play Store, F-Droid, or GitHub (as open-source). PhotoPGP uses the openpgp-api service of OpenKeychain for secure key handling and cyptographic support.
If you have the PhotoPGP Full Edition, you will need to import public-keys into OpenKeychain for anyone you want to authorize to decrypt specific photos.
There are many ways to create your PGP private key. You can, for example, create your PGP key using Windows or Linux, then copy that key to your phone.
This screen shows when OpenKeychain has imported the public key.
Click the X to return to PhotoPGP, or click the back button.
This screen shows when OpenKeychain has imported the private key.
Method 1 - use INDEX tab "ADD PHOTO" icon: