How do I turn off MMS messaging on the iPhone?
Sometimes a multimedia message or picture message (MMS) will cost more to receive for prepaid wireless subscribers. Some devices won't even accept an MMS message. The iPhone will attempt to send an MMS in certain situations, such as when texting multiple recipients.
So how can you be sure your iPhone only sends SMS text messages? You can turn off MMS on the iPhone by following these steps:
Navigate to Settings -> Messages -> MMS Messaging -> OFF