I tried to google what it was all about, as well as search on TL and have had no luck, so I decided to start a topic seeing if anyone knows what this is about.
The requirement for the portrait is to 'Kill a stank before it kills anything on Hard difficulty'. The reward is a portrait of a Stank.
It looks like this could be something from some type of Zerg mission, was just wondering if anyone had any thoughts on this.
Here's a screenshot to prove that this is real!
Also: Sorry if I broke any rules, first topic I've ever posted :o
Edit: Wow! I'm completely oblivious and just realized that there are 2 other new portraits as well! It seems like you unlock these portraits from playing custom games.
There's also a new portrait of the "Aiur Chef" and one of Ornatus, which you unlock from playing Starjeweled.