Someone should set up a web service for sending MMSs. This is how it is envisioned in my mind...
You sign up and get a unique email address assigned to your user account, when you sign up for the account you put in your name and phone number. Then when you send a picture to this email address from you use the subject line to put the phone number of the recipient, and you can put a message along with the picture in the body. When you hit send the service sends the body of the email (picture and message) to the mobile number (from the subject line) using each of these email gateways, since the phone number is only on one service only the phone intended will get the message. Also, since the service has your phone number it will automatically put this message at the bottom "to reply to this message send a text to <user's phone number here>"
You sign up and get a unique email address assigned to your user account, when you sign up for the account you put in your name and phone number. Then when you send a picture to this email address from you use the subject line to put the phone number of the recipient, and you can put a message along with the picture in the body. When you hit send the service sends the body of the email (picture and message) to the mobile number (from the subject line) using each of these email gateways, since the phone number is only on one service only the phone intended will get the message. Also, since the service has your phone number it will automatically put this message at the bottom "to reply to this message send a text to <user's phone number here>"