Can only send photos in iMessage

For the past week I cannot send photos as a mms to any of my contacts. I was able to get a photo to go through iMessage to a couple of iPhone friends, but can't get anything out to anyone else; my phone tries, and then I get the red exclamation point and "message send failure". I can't figure out if it's my phone, my carrier or apple.

I'm using an iPhone 4s, latest iOS, on Verizon.

Thanks!