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

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.