Let Me See Your Face

Okay, this is officially my second critique ever, so bear with me here.
I want to applaud your use of the Avatar characters here. Not only do they look like themselves, but they ACT like themselves. In my opinion, fan art is most effective when it accurately represents the characters in their body language, pose, expression, etc, in addition to their looks. You have here not only captured each character, but their dynamic as well. Toph, ever the prankster, and Sokka, all too often on the receiving end of jokes (not just from Toph).
You've successfully painted a full picture here. The redness on the bottom of Toph's feet necessitates her need to sit down. Sokka, squatting down next to her, likely in an attempt to help in some way. It all really helps to make a scene, rather than seemingly arbitrary posing.
The yellow in gradient background compliments the figures (through the yellow accents in their clothes) and the green in the gradient serves to contrast to their overall warm colours as well as an "anchor" to ground them. The fact that they cast a shadow helps with this as well. Too many artists, my self included, would have overlooked or skipped the shadow, but it really adds to idea of them sitting on something.
The only reason I'm taking off a half a star is because I personally feel there is too much dead space. Perhaps the text could be larger, or it could be more closely cropped?
Overall, I greatly enjoy this picture. You get a real sense of the characters and their relationship, even if you aren't familiar with Avatar.
PS- LOVE Sokka's face. Even though you only see one eye, it says so much!