To most, that question seems to beg the obvious. And I hope I'm not the only one who has thought of this. But I don't think the picture seen above is as obvious as it seems.
Most see the image and automatically assume that Tony Stark has taken to the Hulkbuster armor to battle a crazed HULK. This brings up all sorts of trust issues. Just the fact that he built an armor just to fight the Hulk makes Tony and Bruce's relationship a little strained. And, no matter what is going on in the picture, they will have to deal with this. The fact that they spend so much time building these two as the 'science twins' who are joined at the hip leaves us wondering if they aren't heading to a show down in their bromance. Obviously, the HULKBUSTER armor's very existence will make it extremely hard for them to just move on as if nothing is going on. Not only is his teammates, but his closest associate, Tony, is plotting against him. What's worst, they have built something to 'deal' with his angrier side. Like I said, TRUST ISSUES ensue.
But, forget all of that for a minute. Look at the picture again?
What is actually going on here? Is this a picture of Tony and Bruce having it out? Really? Are you sure?
Here's another option that I haven't heard ANYBODY bring up. What is the possibility that something has taken control of the Hulkbuster? Something that can control the armor...or infected it?
Ok, here it is in a nutshell (although most of the true believers out there are already ahead of me). The movie's about ULTRON; an artificial intelligence who is constantly upgrading itself trying to find perfection. What I see in this picture is possibly a Hulkbuster armor taken over by Ultron and used against the Avengers. This may be just another creative way of playing out the whole Ultron saga where the growing A.I. has deduced the best way to neutralize the Hulk. If he is truly created by Tony Stark, he would know of the armor's existence and purpose. SO, the way I see it. This is an even more viable option than the 'obvious'.