I read in the comments to another thing that Sakari is supposed to be albino. Issue one is that if he were albino, he would have white hair and pale skin as he does, and his eyes would be red. Issue two is that albinos don't pass down blond hair to their children, and seeing as Nilak's mother was most likely dark-skinned and dark-haired, which are dominant traits, it's much more likely he'd have dark skin and hair. Also, Polar Water Tribesmen keep their hair in braids, ponytails and topknots, the kind on Sokka, the man in your background, Hakoda, Bato and in LoK, Unalaq, Tonraq and Tarrlok. Issue three is the gem; the Poles are made of a layer of snow, on ice, on water, on rock. And in the rock, there isn't much of value, except for building materials like limestone and fuels. Issue four is that Polar Water Tribe children don't wear shorts or short boots.
Now, leaving aside what I don't like, let's get on to what I like in this image. I like the detailing on their fur clothing that makes it stand out as rough animal skin, and the fact that Sakari's sleeves look to have been made and sewn on to his shirt from a separate skin, as the Polar Water Tribes would hunt animals and skin them themselves for clothing. I feel the Water Tribe logo on Nilak presents a need to show his Polar Water Tribe heritage, because it's not something his features do for him very well. It's almost as if he's showing the people around him that he is 'one of them', even though he looks different.
On a non-picture related topic, how did you choose names? 'Sakari' seems very Eastern for the Polar Water Tribes, and 'Nilak' seems very Nordic for the Polar Water Tribes.