Is iMessage on the iPhone 4s free?

I have unlimited texting but i don't have internet i only use the Wi-Fi at my house my mom blocked my internet so I can't use it without Wi-Fi. If I send pictures to other people with iPhones is it free or would I need to pay extra?

IMessage is free, but it uses data, so unless you are hooked to a hotspot, its using data from your wireless company (which you said your mom blocked)

Its free but if you want get a router hook it up in your room hide it from your mom.

Imessage is always free.

Do I have to download iMessage on. To my iPhone?