I agreee with the weak signal theory.
Thing is, this also occurs on Wi-Fi. Try to send once a first picture, goes no problem. Try a second pic, fails. Reboot, retry, works ! Try a second picture, fail. Rince and repeat.
Same with 4G/LTE.
Ok... so I did a full factory reset, installed Facebook only right after all the system's updates, tried and I still have the same bug.
First message with picture goes without any problem.
Second fails. Sends after a restart.
Very annoying.
Wow ! Sorry I can't help, but I'm glad I'm not the only one with this issue.
Also on my Telus Galaxy S3.
I usually reboot the phone, then re-send and 95% of the time it will work. But then I'd try to send a second picture, and BAM "Your message was not sent" once again.
I've had this...