That picture was being spread around on the Steam forums before valve made that post.
There were tens of thousands of posts on the steam forums of people liking the dog idea, so I'm assuming valve made that post on their blog as a reply to to posters on the steam forum.
So yes, the picture is fake and fan made, but people liked the idea, so I'm assuming valve will look into it as a possibility, but at this point I'd assume almost no work has been put towards that, so it is not an impending update, just a fan idea that has caught a lot of attention.