MMS is disabled on some iPhones. To enable it go to settings, and go into the general settings. From the menu select network and select cellular data in the network menu. In the Cellular Data settings go down to MMS options. Now set the APN as wap:voicestream.com, MMSC as http://mms.MSG.eng.t-mobile.com, and MMS Proxy as 184.108.40.206:8080. Once you do these settings go to the message settings and make sure that the MMS messaging is turned on. Now go to messages and create a new message. Add picture to it to make it an MMS messages. Use you gallery to import a picture to the message. You can also add a video to it. Now send the MMS message and test if it is working. This video shows how to enable MMS on iPhone.