MMS messages are completely free with all SMARTY plans.
Information about MMS messages
Sometimes, your phone will have to convert a text message into a different format – an MMS. MMS stands for Multimedia Messaging Service. If this happens, the text will still be included in your free text allowance.
However, just so you know, an SMS may change to an MMS if:
Your message is too long to send as a normal text message.
A text message is sent to a large number of contacts.
You attach anything to a text, such as pictures, music or other files.
You include emojis within a message.
You try to send a text to an email address.