If cyphertext in image metadata would be acceptably well-hidden, this should be as well: you can hide four bits per pixel using the least significant bits of the RGBA channels. A human won't be able to tell the difference between #FE007BFE and #FF017AFF.
(This assumes .png. I don't think it would work very well, if at all, for either jpeg or gif.)
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.