Bumping this. My whole family has this issue. We are on various networks. I personally am on AT&T but any time I try to send a iMessage photo it tells me it failed to send, but in reality the other person does receive it.
There are about 7 of us all with the same problem whether we send to a group or individuals. I've done a hard reset, network reset, etc.
I have this same exact issue. I'm on AT&T too.
But here's the catch, do you have another iMessage device associated with your Apple ID?
I know what my problem is, but I haven't figured out what causes it, or an permanent fix.
For example, when I would try to send a picture to a group iMessage from my phone, the picture will send but I'll get the "Not Delivered" notification not too long after. However, everyone in the group would have received the message. Here's where the problem comes into play. I would go to my MacBook, and iMessage would have updated properly with the picture and all in that conversation. ON THE OTHER HAND, my iPad Air would just show the strictly text messages within that group, and wouldn't pick up pictures, videos, etc. That I sent or other people sent me.
The only way I know of to alleviate this issue is to restart the iPad and it will start working again for the time being. As in everything will start delivering properly; to your friends' devices AND yours.
The problem seems to reside in keeping all the iMessages in sync across multiple devices. I really haven't figured out what triggers it seeing as everything will be sending fine, then the next moment I'll be getting the "Not Delivered" notifications.
It's a really annoying issue that's been on going for like 2-3 months now so I'm assuming it's something on Apple's end, or even iOS 7.1 related. I don't think I had this issue before the 7.1 update.