If I read this right, you said Optimus S. The "S" is the Sprint phone, not the MetroPCS phone. It is a Optimus M.
So if you have the "S" and it was flashed to metro, their my be a change in metro's system for using your MMS to send pics.
It's easier to just use gmail and send a email to the phone number email@example.com as a attachment.
try it that way