How many pics are you trying to send? Are you sending to a #? or an email address?
Can you verify that you have these settings in place?
Go to Settings, General, Cellular, Cellular Data Network:
MMS APN: epc.tmobile.com
MMS Proxy: 18.104.22.168:8080
MMS Max Message Size: 1048546
MMS UA Prof URL: http://www.apple.com/mms/uaprof.rdf