you got the characters down, you understand proportion and expressions, but what i think would make your pieces even more impressive would be to place the characters on a solid plane, like including a wall and a floor ect. the shadows confuse me a bit, the shadows to the left of bat girl's head suggests the light source is in the front casting a shadow on the floor on the back of her head, but the gradient background and the shadows on her arms give the sense the light source is from the back so there's some conflictions. You're getting there, these are things you should keep in mind when you're developing a piece.