Open How to send MMS message with 2 images

I am attempting to use Join to send & receive MMS messages with up to 3 images in a message from my desktop.
I have tried Join on the Web but it does not seem to support MMS.
The Chrome extension works for a single image but when I attempt to add a second image it overrides the first one.
How can I send an MMS message with 2+ images?

My environment .. Moto G phone - Android 5.1 and Desktop PC - Windows 8.1, Firefox 57.0, Chrome 63.0