If PJP's posting tutorial doesn't work for you(it doesn't for me) ,then on the toolbar above where you type your reply,there should an "insert/edit image" button(looks like a tree) if you click on that,you'll get a pop-up box appear, you then copy your image url into the "image url" field at the top of the pop-up box then click on the "insert" at the bottom your image will appear.
EDIT-ooops just noticed your post above...