Actually i had the same issue when i first got my i760. i returned the phone and got a new one with the same issue. then i spoke to a verizon tech friend of mine and he fixed it. he added a broadband type access to my phone with #777 for mms messages. go to connections and add new.
Login:
yourphonenumber@vzw3g.com
Password: vzw
Should fix the problem.
i do however have unlimited txt and pic messaging. so that might affect your bill somehow if you don't.