Images are just the lowest hanging fruit and attract a lot of sloppy schemes. You can encode information in pretty much anything as long as you are free to choose the message.
You could even encode info in plain text by the lengths of non-whitespace characters, modulo 2. As an example, try using the so-defined scheme on the letters count of this very sentence; they're encoding, repeatingly, S.O.S.
Posts of similar length as this one are enough to send a public key using ECDH and negotiate a shared secret for use with a block or stream cypher. You could then send short messages using this shared secret inconspicuously. Shannon entropy of English is about 1.5 bits per character so you could store quite a bit if you cared to compress it well before encrypting, preferably using a shared static dictionary.
