[ossguy]

https://jmp.chat/ supports MMS and concatenated SMS.

JMP has full support for sending and receiving picture messages, and can receive group texts as well (which Twilio doesn't support). Sounds like JMP is what you want for an SMS-over-XMPP solution.

You can even port in your number from your existing VoIP carrier - see https://jmp.chat/#existing for details.