First of all, I feel younger when I look at this drawing, and I like that another comment is how creative this is, but for constructive criticism, I have one thing to say. It is the color. I feel that it may have been unfair to say that, but I am trying to help by finding a flaw. All the colors are white, blue, green, and black. It is good, but if it had a colorful background, there would be an impossible chance to find something wrong in this picture. Good work though keep it up. I will be looking forward to your drawings
Pues creo que no es tan difícil, pega una textura que te guste encima de tu imagen y le cambias las propiedades de las capas, por ejemplo, Overlay o Screen, todo depende de cómo lo quieras. Te mando saludos y sigue practicando!