You can improve, but to be really good at art you just have to be naturally good at it.
I remember watching a program a while back where this boy had learning difficulties and wasn't good at school but could draw pitcures that were perfect. They took him on a helicopter ride over London and they took a photo at one point and when they landed, without looking at the photo, he drew a picture exact to the photo. It was incredible to watch. He was selling his drawings for thousands, and he was learning to paint. If he masters paint then he could make a fortune. All because of this natural talent.