Here is a solution that came up on the Apple Support Forum regarding this issue:
"Well I spent a hours on the phone with Apple care and the guy submitted a ticket to the higher ups. Nothing ever came of that though. I figure that the iphone 5 has to be sending pics/videos through a different port or with added security and the xfinity gateway blocks it.
I called Comcast yesterday and the rep was trying to turn my gateway into just a modem, but it wouldn't listen to her commands. I decided that it's not worth the hassle and am returning the gateway to comcast. I went out and bought a stand alone comcast modem for $99 at best buy, but if you don't want to do that, you could always just exchange the gateway for a separate modem and router at the comcast office. My parents have comcast and they rent their modem and router from comcast, but the router is netgear and is a separate unit.
So my current set up is a comcast cable modem: Motorola Surfboard docsis 3.0 high-speed cable modem and an older b/g netgear router that I had in the closet. Everything works great and my iMessage can now send photos and videos with no issues. The Xfinity gateway is the issue, at least for me it was."
I'm guessing that this may be the issue that quite a few of us are experiencing. I hope it helps!