What if the images are entirely like the text, various scenarios.

[MichelleL11]

Michelle: out of my (2) follows that this possibility is actually covered by (3)

Ah, OK - all ciphers in one class - got it.  I would then observe that the reason for the ciphering would impact the kind of imagery that would accompany it, then.

Like suggested above, you would perhaps expect a different kind of parallel imagery depending on whether the ciphering was done to escape persecution (quick thought: if you’re that concerned about detection, l would argue no imagery would be a smarter move - it would definitely get less notice, although maybe be an imperfect “expression”).

But if the reason for encoding is to allow access only to the “initiated” and the imagery followed the same pattern, l agree with JKP that hiding the message would be expected.