All you need to do is encode and post one picture with a hidden message, decode one message from one of your fellow students. If you do it more it will a good practice but will not add to your grade. That means do not get hung up on one “encoded picture.” Try next one. If somebody was not able to decode your message, first make sure at least you can do it. Post the passphrase you used to hide the message. Try to keep the default settings. If not post the settings you used. Try to make it easy on your fellow students. If nobody can decode your picture then the onus lies on you to provide instructions and provide reasonable help.