2017 CX-5
I use tapatalk and there is a button for it above the keyboard. I believe tapatalk is hosting it. Otherwise you’re going to have to host it someplace and use an html snippet in your post. A lot of the image hosting places will generate that for you.
2016 CX-9 GT AWD
You can click the image button and load the URL or when you upload an image to a website like imgbb it will give you the html code and you can just copy and paste it into the message here


2018 CX-5 GT FWD Premium
Its a pain but here's how I've been able to do it.

Manage Files
Choose File
After picture is uploaded , click the blue hyperlink
The attached file (picture) should open in a new window
Copy the url
Click the Insert Image button
Copy and Paste URL


On a related note...We should have improved image / file hosting functionality and features in the future after we move to XenForo...Stay tuned! :D