Usually, MMS is dependent on a data connection. That's the first line of troubleshooting. Make sure you are connected to your mobile data or wifi before sending a picture. If you go out of a signal area, your pictures won't send and you won't receive either.
Do a battery pull as a secondline troublehshooting.
Lastly, do a back up of your data through your gmail acct and then do a factory reset. This is a last resort.