Google just recently enabled MMS support for North American Google Voice users, but it looks like the service is already seeing its first hiccup. Many users are reporting being unable to send any MMS messages, and Google has acknowledged that its a problem on the official support site. Fortunately, SMS messages seem like they’re okay, so if you’re solely using a GV number, you’re not totally up the creek until this gets fixed.
Hopefully Google will get the problem sorted out sooner rather than later, but if you want to keep up with the progress, you can find the thread below.
source: Google Forums