Ughhh. I have a headache from researching this issue all day.
My conclusion as of 12/30/09 is that we can configure HTC Messaging to recieve from Sprint, but no luck on sending as of yet.
If you are like me, you do not want to disable HTC Messaging and deal with the crappy Arcsoft Software instead, so as a temporary solution for sending and still keeping HTC Messaging for threading, use sMMS Picture Mail Composer for Sprint:
sMMS v0.8.3.14000 - Sprint Picture Mail
http://forum.ppcgeeks.com/showthread.php?t=22509