When I received MMS messages on my very old phone with no mms support, they arrived as SMS messages with some kind of embedded url to a server at the provider's. I guess that's what MMS messages really are: A url embedded in an SMS which your phone then goes out and downloads through GPRS.

Personally I've never sent a single MMS in my life and there's no reason to believe I ever will. I send lots of pictures by other means though.