Here are the CORRECT settings to send AND RECIEVE pictures on an iPhone 3g 3.1.3
It took me a few hours to finally get this right!
MMS Proxy: 188.8.131.52:8080
MMS Max Message Size: 1048576
MMS UA Prof URL: http//www.apple.com/mms/uaprof.rdf
Then go to Cydia and add this source:
Then in Cydia, go to Sections, then Tweaks scroll to T-Mobile US MMS Fix (its the 3rd one) and install. It says for iOS4 but it worked for me! I've tried the other T-mobile tweaks and they didnt.
I can now actually RECEIVE pictures!!! YAAAAAY! Happy sexting!