Yeah... There is a trick to Straight Talk MMS.
AT&T style SIM, yes?
Unfortunately I am not up on the menu trees for WP 7.x... Your MMS settings should be right in the area where you found APN. On WP 8.x it is Settings > cellular + SIM > SIM settings > edit MMS APN (your path may differ).
So for ST on an ATT style SIM, the trick is that "WAP gateway (URL)" has to be translated from a http style address to an IP address format.
MMS settings need to be:
"APN" tfdata
"User Name" <leave blank>
"Password" <leave blank>
"Authentication Type" should be set to PAP
"WAP gateway (URL)" must be 66.209.11.33
"WAP gateway port" 80
"MMSC (URL)"
http://mms-tf.net
"MMSC port" 80
"Maximum MMS size" <leave blank>
"IP type" IPv4v6
Hope this helps...