IPhone 4S only receiving picture messages through Data?

I'm only receiving picture texts when my Cellular Data is turned on. I want to be able to receive them through Wi-Fi
Why is it doing this? And I already have MMS enabled

Picture messages doesn't use Wi-Fi. It uses ONLY the cell networks.

1. If you send a picture, it'll use the cell network. It will NOT send pictures threw Wi-Fi.

2. When you receive picture messages, it'll use the cell network to receive the picture.

They do this because not all phones uses Wi-Fi. And the phones that don't use Wi-Fi, they use the cell network. There's no way to change this.