iMessage: Picture shows on notification, but disappears in conversation.

I started having this problem with iMessages where I will be notified that I received a picture message and I will see it in on the locked screen, but when I open the conversation nothing is there. This happens sporadically, so I am able to receive some pictures. After I restart the phone, I get an image of "? Unknown" in a comment bubble where the picture that didn't come through should be. As far as I am aware, this just started happening yesterday, and it is sporadic. Some images come through while others don't. Again, I am able to see the photo on the locked screen or the notification of the message, however, I am not able to see it in the message.

I am on a iPhone 4s with iOS 6.0.1.

I have read somewhere that it may be an unknown file error, however, the sending party (my girlfriend) is using the same version of iPhone and using only the built-in camera app.