Good shading there in the pic. Like the folds in Zaheer's robe and general stances of the characters. Some parts have a few odd proportions such as Zaheer's ears not having enough detail (had a more rounded and chewed up appearance) and the fire being a bit too blurry, but aside from that good pic :).
For me, I liked that the rebuilding of the air nation, it was neat seeing Korra and Asami with a more fleshed out friendship, Lin overcoming her issues with her sister Sue, and many of the intense action moments. The show definitely has great visual execution, both in action and emotion, especially seeing Korra immensely weakened from the poison (I'd guess it's mercury since it was made of metal).
My issue with book 3, is just that there really hasn't been that much change or development in the characters. They have moments where they do something incredible, but there doesn't seem to be any growth in them, like Korra still feel like she hasn't move on from being aggressive and showing more consistent restraint. She's still likable, but nothing much has happened with her character. I also kind of found the villains, while effective, lacking some depth because while their cause is interesting, their motivation isn't addressed very well for them. What caused them to go against world leaders, how does the rest of the red lotus function (the ones outside the main 4), what influenced Zaheer to be an enlightened airbender and how exactly he became so good at it. An episode with backstory, similar to the one with Tarlock and Amon, probably would have been helpful.
Maybe book 4 will give more context to all that, just wish it was all completed in this season since the creators stated that each book will generally be their own story. Ah well, maybe I'm being to harsh for the criticism. Enjoyed what was given and curious to see what book 4 will focus on.