The image is incidental. What Fable is actually doing is generic encoding/decoding of a message into a medium. Ask it to hide a message in a poem and it should try to apply those encodings into the new medium. Fable could use quantized, margin-rich encoding tailored to whatever features are available in that medium.

Let's see if we can stego secret messages into SVG pelicans on bicycles, as a test.

Will be interesting to see agents intentionally hiding this stuff in a way operators can't spot from any human readable output or thinking trace, evading detection. Wonder if this has happened yet, but it seems inevitable.