I want to improve spriting and am looking for constructive criticism

So I make sprites online and I am decent and I want to improve. I make sprites of sizes generally 64 x 64 and higher. Any constructive criticism is great.  I have examples of my work below. I have a deviantART as well with a hundred other sprites at http://driedmanjello.deviantart.com/ Thanks.

  Alphamon.png (Size: 1.99 KB / Downloads: 26)

  Courage the Cowardly Dog.png (Size: 1.96 KB / Downloads: 25)

  Squash.png (Size: 1.78 KB / Downloads: 25)

  Boltmon.png (Size: 1.66 KB / Downloads: 25)

  Beelzemon_Dark.png (Size: 2.06 KB / Downloads: 24)