If you've got T-Mobile as your carrier, and you have an unlimited texting plan, check out this video to learn how to enable MMS on your iPhone. Assuming you have this plan, you should be able to send multimedia messages.
Here are the settings needed:
Username: leave blank
Password: leave blank
MMS Proxy: 188.8.131.52:8080
MMS Max Message Size: leave blank
MMS UA Prof URL: leave blank
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