You need to use the [img ] [/img ] tag at the top of the "post a reply" window. And between the two img, you copy the link of your image, that you can obtain there http://www.freeimagehosting.net (for example)
Or ... at the bottom of the "post a reply" window you have a "Upload attachment" tab.
That should work. Is it what you needed ?
There's a bit of magic in everything and then some loss to even things out