Sending MMS via Mobile App
To send a multimedia, use the camera icon to the left of the SMS text area on the mobile app.
This will bring up a dialogue box that will allow you to create an image or video, as well as select one from your libraries on your phone.
Sending MMS via Web App
To send a multimedia message, use the camera icon next to the "send" button.
If you're on a regular computer, this will bring up a file picker where you can select a file to send.
If you're logged in on your phone (not in the mobile app), it will allow you to access your camera or your photo library instead.