This guide will let you setup your iPhone for wireless data and MMS (picture messaging).

(From http://smartphonematters.com/guides/how-to-set-up-straight-talk-wireless-data-and-mms-for-your-iphone-running-ios-7/)

NOTE: Follow the guide EXACTLY as described. There is a timing process for switching between the Cellular Data Network settings and the Notes app (using multitasking). This allows the Cellular Data Network settings page to stay open even after the T-Mobile SIM card is removed, and the Straight Talk one is inserted.

APN settings

The settings I used:

APN: tfdata
MMS APN: tfdata
MMSC: http://mms-tf.net
MMS Proxy: mms3.tracfone.com:80
MMS Max Message Size: 1048576
MMS UA Prof URL: http://www.apple.com/mms/uaprof.rdf