Foil Pics allows you to encrypt some pictures from the gallery with a password stronger than the lock code. Strictly speaking, each picture is encrypted with a unique random 256-bit AES key which in turn is encrypted with an RSA key which in turn is encrypted with your password. If the bad guys get your encrypted pictures, they would have to crack the AES key (different for each picture) or the RSA key (shared by all pictures but harder to crack) in order to extract the content. If they get the encrypted RSA key as well, then they can brute force your password. So in the end, the encryption is as strong as your password.
The format of the encrypted file is described here.