Are you using LG Stock, or AdvancedStock7? Also, with either one, if you do not use the LG Messaging application, you need to manually set in the app (If the app has it) the MMS settings to use. Please refer to my ROM's Thread OP (
[ROM] AdvancedStock7 an LSZV7 Based ROM - Android Forums) for the Proper MMS Settings for Virgin Mobile USA.