Nice work, good job in incorporating the thumbnail. Nice colors, although the sky could be a bit more saturated. As Savory said, the arms are a bit short but otherwise good job on the character. I would have liked it if it had a bit more shading other than just the thin strokes along the lines. That could really make the character a bit less flat.
Another point of improvement in the drawing is the perspective on the ground. Looking at the grain/grass you drew in front; the horizon should be a lot lower, and it probably shouldnt curve that much. As it is now it gives the idea the the foreground (character/grass) are seperated from the background.