It’s just a bug that enables people to load textures from the game files into chat messages.
There’s not much “hacking” involved. We’ll probably see more and more of that stuff when more people figure it out, until it will eventually get fixed. While I see that this can get pretty annoying I don’t see any way how this could be abused in any way that would harm players (apart from the annoyance).
I filed a bug report and guess this will be fixed soon.
Edit: This is not caused by addons. As far as I can tell the reason that some see links and some see images is probably just due to the chat frame size being to small to display the image.