As I understand it, encryption works through software using a given algorithm to convert a given (readable) set of data to an unreadable form. The person encrypting has a key they could use to decrypt the data into a readable form.

Could software be written with the following functionality? I specify the jpeg I want to encrypt (say a picture of me in leather with a rose between my teeth which I use as my profile on a sadomasochism site), specify the encryption algoritm to use, specify the key, and lastly specify a decoy jpeg (say a pic of a rose).

The software then provides me with an encryped file which can be decoded to the original using the key I specified, and it also provides a key which, if used, will convert the encrypted file to the decoy jpeg.