According to the help message at the top of the screen, you simply
I've tried this in the past with something like:Insert image: [img]http://image_url[/img] (alt+p)
but it just don't work (or at least it doesn't work in preview).
Be assured, I'm not intending to overload the site with images - would simply like to know how to do it for the odd occasion that it would be appropriate.
Thanks.