It's not just you guys. I have a GS3 and I occasionally get the same thing. While most picture messages are sent to me from my wife and friends (mostly all using iPhones), it only happens about half of the time. I've come to the conclusion it does that when they send an MMS a certain way. I think if they send the pic directly from the messaging app, it comes as a pic. If they send the pic from the gallery first, it sends as a video (to accommodate multiple pics in one message). That's the only thing I've been able to come up with.