Something very similar was occurring with my phone so I called T-Mobile's support line. The guy walked me through some simple steps and my pic messaging started working again. I'll walk you through them to see if it helps. (Don't worry. None of these steps will change/erase your phone or saved info/pics/music)
1. Go to settings > wireless and network > mobile network
2. Make sure both 'Use packet data' and 'Data roaming' are selected
3. On that same page go to Access point names
4. Make sure 'T-Mobile US' is selected
5. Go back 3 times where you will see the settings menu again
6. Go to applications
7. Once in applications go to 'Manage applications'
8. Go to the 'all' tab and scroll down until you are in the M's section
9. Find and click 'Messaging'
10. Go down to the 'Storage' section and click 'Clear data'
Send a picture message to yourself or to someone else to see if it works.
Hope this helps. If not, try calling 611. They're actually quite nice sometimes.