I just put my sim card into another phone and changed my text plan from iPhone Unlimited Texting to Unlimited texting. They are both $20 a month. When I put my sim card back in my iPhone I sent and received picture messages.
It hasn't been working for me, but I did put my iPhone sim card in another phone and sent a few MMS messages (the first one failed, but worked after that) Then I sent a few more to myself to make sure I was getting them. I put the sim card back in the iPhone (without doing anything with my account). The first try failed, so I did a hard reset and it worked. MMS is SLOW!!!! (it looks like it is going to fail but it went through after about 20-30 seconds.