Desperately seeking how to send photos via messaging

How do I change the APN on my iPhone 4 (7.0.4 system) so I can send and recieve photo messages? PLEASE is there ANYONE out there that knows how to fix this? I just had my iPhone unlocked so I can use it and NOW I can't find where I change or make sure the APN settings are correct!!! I've been waiting for months to get this phone and NOW it won't send pics via text message!!! Uggg Desperately seeking answers.