All you have to do to avoid that is NOT have your Japanese penpal's international number in your contacts. Just his email address. Then the message app can't send anything but an iMessage via his email.
Orange have just charged me for sending a message to an Ipad, I only have an email address for the contact, no number as it doesnt even have 3G! So it was never going to receive the message. It contained only text, no emoticons or even smileys.
It there is an option to turn picture messages off with Orange I will use it as I use whatsapp for just that purpose.