Hi, I am New to the community, but I just had to share what fixed my mms problem. It took me about 4 days, but I figured it out.
Hopefully it will work for you.
The fix for mine was to reset the APN.
I did the following:
2. Tethering and Networks
3. Mobile Network
4. Access point name
5. Hit menu, choose reset to default
6. exit out
7. Try to send a picture message
8. If works great if not restart phone and try to send mms.
Hope it helps.