You could use an image. But you could use text as well. E.g. you could agree on a code phrase to be said when some "dirty deed done dirt cheap" has been completed. Or you could encode a binary string by alternating British English spellings with American English Spellings: e.g. "color" means 0, "colour" means 1; "gray" means 0, "grey" means 1, etc etc. and then just use those alternate spellings in a normal conversation.
The problem with codes is you have to remember them. And then you'll need a massive lookup-table. People don't want to have chats based on limited vocabulary.
This is why we have modern encryption. It converts the most beautiful poem in the world to complete noise and back with no loss of meaning. It allows sending images, books, videos -- culture, without spycraft that requires hours of learning. It's also more secure, given that humans aren't nearly as good at coming up with randomness and a computer's hardware RNG.
But then it is obvious you are using encryption. A lot can be learned just from the timing of the message and noting it is encrypted. I always thought it best to send an encrypted message every hour of every day .. most messages would be just nonsense. The one eavesdropping could not only not read the message, but they would gain no knowledge about when the "real" messages are being sent.
Yeah that hourly packet rate is called traffic masking or traffic flow confidentiality. It's offered in some instances of IPSec, but given that it's not exactly a common protocol in end-user products you don't really see it. I did implement it for my messaging system however https://github.com/maqp/tfc/wiki/Commands#14-traffic-masking
