This might be more than you asked for but here goes: Cellular networks still have to interop with SS7. On most major large Cellular networks this is accomplish

This might be more than you asked for but here goes: Cellular networks still have to interop with SS7. On most major large Cellular networks this is accomplished with a protocol called IMS. Integrated Multimedia Subsystem provides wrappers for non-IP transactions to allow IP controls over analog equipment. Put simply: stuff that isn't IP gets broken up and wrapped in IP before it can go on the network. Because of this, and other factors, making a picture messaging system that operates over this structure is difficult. So difficult that virtually all of the MMSC (MMS Gateways) are proprietary and prohibitively expensive. Why doesn't anyone support MMS? Because it costs too much.