If I sent an encoded picture message, it would have to be one of two ways:

1. A pre-arranged key is used to decrypt symbolism that is likely not visible upon glance, other than maybe a single confirmed oddity that there is more to be seen

But that only really works for pre-arranged messages. You can create messages from a pre-shared list, but that doesn't give you enough creativity if your message is something completely unexpected.

2. Symbolism, multiple confirms, a number, and confirmations on a post related to said number.
Or other similar messages.

It seems #2 we can decode through study.
Gematria also works, but that code is known thanks to POTUS, and tends to only be used to communicate to them, or to them and others (you guyz).
No back communication lines anymore. Pre-arranged messages would have run thin, and symbolism will be their only way left to communicate.
The choice to see will be yours.
Have fun with it!