To encrypt or decrypt current image,  select in the main menu -> Image -> Encryption -> Encrypt or Decrypt.

The function encrypts/decrypts the current image using the Tiny Encryption Algorithm with a key of 128 bits. You need to specify the password key for encryption and decryption.

Warning: The image must be saved using a lossless format and in full rgb color spaces (no palette). If you use a lossy format such as JPEG, or a limited color format, such as GIF, you will not be able to decrypt the image. There is no way to know whether an image is encrypted or not.