Sending MMS from an ATT phone is fine. The reason the MMS looks horrible on a TMobile iPhone is because TMobile doesn't recognize the phone. When an MMS comes across the network to a phone TMobile doesn't know they automatically re-size the image 160x120 then the iPhone stretches it out to make it even worse.
The "fix" for this will have to be some way of making TMobile believe you are on a Nokia or something else. Then your MMS messages will look just fine.
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