This is basically using the picture as a One Time Pad, http://en.wikipedia.org/wiki/One_time_pad it can provide very secure encrpytion so long as you don't do anything silly like include the picture along with the encrypted file. If you just agreed with your recipient in advance that you would be using that picture file that the recipient also had a copy of and you only ever use it for one message sent between the two of you it should be secure.