STRAIGHT TALK, iPhone 4 picture and video messaging not working?

I just switched from AT&T to Straight Talk for the everything unlimited plan. However, I can't send picture or video message and the internet is really slow. I called Apple and they tried everything to help, but nothing work. They suggested I should Google for help. Can someone please help me? Thanks.

Added (1). I called Straight Talk they couldn't help either. My phone is already unlock by At&t.

Straight Talk uses T-Mobile's network for GSM phones. T-Mobile uses a different frequency for 3G data than AT&T. Picture and video messaging is largely incompatible as well. Nothing can be done to fix the problem.

Straight talk uses both att and t-mobile. You just need to update your apn settings. On a phone thats jail broke, you have it made.
follow instructions for installing cydia which is a jailbreak app store. Then install tetherme app - free i think. And it will set your apn settings for mms.