MMS (picture) message sending is available on a post-paid plans only. To get this set-up, contact firstname.lastname@example.org and the support team will be able to set you up on a rate card and get this feature enabled on your account.
Once the feature is activated, the MMS option will appear in the Message Content section:
1. Upload your image
2. Give your message a subject (non-mandatory)
3. Write your message
After these steps, complete the normal process for sending a message.
Remember - MMS messages are charged at a higher rate than normal SMS messages, so be sure to double check your costs before sending large MMS broadcasts.