Why won't iMessages send any of my photos? (iPhone 5, iOS 7.1)

Question

Hello!
I'm facing a little problem with iMessage. Each time I try to send a photo from my Camera Roll to someone on my contacts list using iMessage, it would say that the photo is "Delivered;" however, after 10-15 seconds, a red exclamation point will pop up on the side of the image I'm trying to send with the word "Not delivered" right under it. When I tap on the exclamation point, I am prompted to either "Try Again" or "Send as Text Message." I've tried both options, but only the "Send as Text Message" option works. I can send and receive regular messages through iMessage and I can receive photos from other people through iMessage, but I cannot send photos at all. SMS/MMS works fine. It doesn't matter if I am on Wi-Fi or 4G/LTE; I still can't send photos.
I checked out the following troubleshooting guide and tried some of the solutions, but still hasn't been solved.
http://support.apple.com/kb/ts2755
I did not play with any messaging settings, so I'm pretty sure that isn't where the problem is coming from. About two weeks ago, though, I tried sending 8 photos through iMessage all at once. The person I was sending it too never received the photos, but I did not get any red exclamation points. I deleted the 8 photos that my friend never received, but that didn't solve the problem either.
I am using an iPhone 5 on iOS 7.1 (11D167). Any and all help is greatly appreciated.

Answer 1

Update for daveeede, amcelwee08, kirbalo, and everyone else who may have this problem:
I'm now able to send photos through iMessage to my contacts (both iPhone and non-iPhone users). It started working Friday night (April 19) when I went out; I was in a part of my city that I usually don't go to. I found a picture on Facebook that I wanted to share, so I tried sending it to someone with an iPhone 4S. Surprisingly, the photo went through on 4G LTE (the person I sent the picture to replied back). To see if this was temporary, I sent another photo the next day to the same person and to an old phone of mine through iMessage, but this time, I was using my home's wi-fi connection. The photo was received in about 10 seconds. I have yet to try if I can send photos through iMessage on my Mac.
I did not play with any messaging settings, but here are three things that I did leading up to Friday night (the night it started working):
1) My old phone was a Samsung Mystic that had access to my Facebook account through ATT Social Net. I checked my Facebook settings and noticed that it was still connected to my account though ATT Social Net ended late last year. I removed it from my account.
2) Those of you who have Facebook Mobile activated on your phones (text alerts for Facebook notifications), try disabling it by replying 'STOP' 326-65 or whatever number that informed you that text alerts is activated. Think of any other services that send you text alerts frequently. Try disabling some or all for a few hours then try sending a photo.
3) I went to the official Apple support site on my phone, tapped on "Phone, Messages, and Cellular Data," tapped on "SMS, MMS, or iMessage not working as expected," tapped on the first option to send my phone's serial number and system information, and then entered my phone number (you can use email also) on the "iOS Diagnostics" window. A bar stating that it was getting information on the health of my phone popped up. I received a text message within a few minutes that gave me instructions on what to do next; I think I entered a verification code on the "iOS Diagnostics" window. Unfortunately, chat and phone help has a fee unless your phone is still under warranty. Thus, I couldn't get in contact with support. However, this was the last thing I did. I'm not sure, but perhaps sending information of my phone to Apple helped? Here's the site I went to:
https://getsupport.apple.com/Issues.action
I ruled out that it isn't any type of hold of some sort on my ATT account. Initially, I thought messaging would go back to normal after my new billing cycle began, but that was not the case since mine starts every 12th of the month. It might be random, but not entirely sure. Is anyone else able to send photos through iMessage now?