^^^ thats why i said use email addresses when sending thru MMS instead. it could be sent to firstname.lastname@example.org
(i know there are already email addresses there, but just as an example).
then your cell phone number wouldnt matter, because the thing that is designating who it is from is the email address you are sending to.
example: i set up email@example.com
as the email address that is to receive messages from my phone. anytime something is sent to that address, then it would be known that its from me (leaving security out of this for now to make it easier).
then all that would have to happen is the server take the image/video from the message/email and puts it into a thread.