I wrote a couple of scripts that embed / retrieve images in the disguised ssh public key container.
There surely is some room for improvement since the script doesn't take care of the payload's validity as the ssh public key at all while GitHub actually tries to validate it somehow before importing it to their database. That said, most of the encoded images, which of course need to be small enough, seem to have been accepted so far.
Run embed.py to generate a public-key like string:
$ python embed.py lena.jpg
Then use retrieval.py to actually get the embedded images: