It's effectively fanart commissioned by the creators. It may come under Fair Use as a promotional image, but I'm not sure. The DVD covers, at least the ones I have, aren't nearly as nice since they don't show all the main characters.
The main problem I see with #1 is that it doesn't include Zuko, who regardless of where he stands on the face/heel chart is as much a main series character as Katara or Sokka (but not Aang, being the title character is powerful juju ).
Indeed. As far as I can tell, that's also a fan-made image; those poses are the standard ones from a GIS and are clearly just overlayed on the background. My vote goes to the current or the book 1 box art.