Sky

Honestly, I am amazed by the rendering of this, given that the artist is a mouse user. You can consider the following as mere nit-picks of mine but it is helpful to remember them anyway.

For one, it is advisable to have only one light source in which all the gradients and shadings will follow. Despite light is usually scattered in the sky, it still generally follows the main light source of it (e.g. sun, moon, terrestrial spotlight). The lighting in the image is not very consistent.

Two, (well, actually given the limit of the mouse, this is actually forgivable. And to be honest, impressive for the artist to achieve this level with said limit) the smoothness of the rendering could use improvement.

Three, the clouds seem to lie on planes angled differently. It's hard to describe by merely words but, you can imagine clouds being some solid-liquid mixture lying on an invisible plane across the sky. Meaning, clouds (the bases of them at least) would lie parallel, or concentric depending on the angle. Basta, the lie on planes na hindi bali-bali, napatagalog ako, di ko maexplain.

That is pretty much it for me...As I said, it's a good piece, and all I said are just details that could further improve it. Also, I suggest to study up different types of clouds and their characteristics. Anyway, good job and good luck~!