Have you ever been able to send one? Or is this a new issue? Have you tried sending a photo to someone else? Did it start after a new iOS download?
You could also try to reset your network settings. This could be a sim issue. Even though your carrier allows it, it may not be set up properly on your phone.
Try making a screenshot of the photo, then send the screenshot. That makes the image smaller in pixel size. I do it some on photos that won't go through. Also, send the photo by itself, with no text.