If it's a picture in the gallery - yours or someone else's - open a new tab to go to the gallery and grab the [ img] address (there's a "copy" function to help you).

Then paste it into your message.
If the pic you want is elsewhere online, click the "insert image" icon... it will paste [ img] and [ /img] , waiting for you to provide information between them.
(Note, I had to add an extra space in "[ img] here to allow it to display and not to be a command!)
Go to the image, right-click it and select "copy image address". Paste that like so:

Now click the "Preview" button and you'll see a preview of your message complete with image.

(I chose a small image, but you can have larger ones!)
All this works in the standard messaging too.
Click the pics to make them larger.