The message is hidden secretly by making very slight changes to the image. When used on Facebook you can receive the secret messages while looking at an image by pressing ctrl+alt+a. That example is just to show approximately how the new image will look compared to the old one.

The whole point is that the message isn't visible, and it's impossible to be able to tell if there even is a message.

The message is encoded in the image by very slightly changing the values of pixels (or some similar method). The message is only revealed with the browser extension

It's not necessarily impossible to detect that there is a message - there are plenty of ways to detect them. The challenge is being able to extract the message without access to the steganography algorithme used, and the passphrase (which could be bruce forced once you have access to the method used to hide/extract - which is a whole other challenge).

The whole point of steganography is to make it impossible to detect that a message is included.

Some people can encrypt their messages and send that encrypted message.

But for some people, in some oppressive regime, encrypting a message and sending it will result in torture. Those people not only have to encrypt the message, they have to hide the encrypted message.

Unfortunately there are very many proof of concept steganography sysems which demonstrate a concept. ("Let's alter the least significant bits in this gif image to hide a message.") These are usually trivially easy to detect, and often easy to disrupt.

This PoC is nice in that it avoids a common disruption - recompression of images - but it's still a proof of concept.

Since it's from Oxford I would have liked to see some kind of Chaffinch integration, and some discussion of how much cover text you need to make the steganographic text hard to find.

EDIT: Whoops! Sorry about Oxford / Cambridge mixup. I've left my mistake in place.

OP here. You're precisely correct, the goal of this project was to demonstrate a PoC of a steganography algorithm which can avoid JPEG recompression and is hence suitable for use in walled gardens such as Facebook.

Also, Chaffinch is a Cambridge project so <sarcasm>naturally not worth integrating with.</sarcasm> But really, this was a single term's undergraduate research project so scope for such integration was thoroughly out of the question.