Originally Posted by
Soeroah
The picture?
You'll need to link me the one you're talking about. The one I'm using as my desktop background doesn't show any Horde other than Sylvanas, and has no indication that she's responsible. The stance she's in looks as much like she's taken aback by the burning as it does her being smug about it.
The first time I saw the picture I'm talking about it looked more like she just came to after being knocked out (most likely as an ambush by a third party wanting to blame the Horde for the action, just like when the Twilights Hammer tried to frame the Horde for killing a group of druids back in Cataclysm) and found the tree on fire, and she's standing staring at it in shock. Apparently everyone else sees a wine glass in her hand and Horde banners all over the ocean, though.
The strongest indicators, which themselves aren't proof, that we yet have are the book where Sylvanas talks about wanting to attack Stormwind, and even that was a specifically selected chapter given to us before the expansion was revealed, and itself could easily be a red herring.