Why doesn't my Straight Talk phone have any APN settings?

I bought a Samsung Galaxy S3 Straight Talk phone from Wal-Mart as you can see in the photo. I can't send picture messages on it, but can receive them from anyone. I know you need to change the APN settings, but my phone doesn't have them. As you can see in the photo, I only have VPN settings. When I go to more settings and mobile networks, I just have the option to turn on and off mobile data and that is on. Is this because it's a straight talk phone? I know it's a little different from a regular Galaxy S3. Can anyone help me fix this so I can send picture messages? Thank you very much.

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