Re: unlimited plan, not able to receive/send picture messages
If you don't turn your phone off regularly, try that first - many times just a power cycle will resolve issues such as this.
If not, have you changed any of your phone's messaging/MMS settings recently? If you have another phone available that is compatible with your SIM card and you know can do MMS, you can try moving your SIM to it to see if MMS works. If it does, it's an issue with the settings on your phone. If you haven't changed any settings, you can't test it with another phone, or MMS doesn't function with your SIM in another phone, contact customer service and have them check the provisioning for MMS on your account - maybe something has gotten messed up on at&t's side.
I'd call from another phone (1-800-331-0500). Once you are transferred to the GoPhone IVR, state "more options", then "customer service" - after a few menu prompts you'll be connected with a live representative.
=^..^= There must be a happy medium somewhere between being totally informed and blissfully unaware.