Honestly, these are so nice for a beginner.
What I would suggest is:
-Look at the edges and learn what antyaliasing is.
-Think of the shape of the sprite when you add light/shade.
-Don't be afraid to use light and dark colors near each other, good contrasts can do magic.
-Last, but not least. Don't give up. Everything comes with time. The more you do that, the better you become.
Glhf