At the bottom of the message compose window (the bit where you type in text) you will see four little check boxes and then section titled "Attach file" with a blank box with a "Browse" button. Click this and pick the picture on your hard drive that you want to include in the thread. Then click OK. Now when you submit reply the picture will be attached at the end of the message. Using this method you can only attach one picture per thread reply. If you want multiple pictures submit multiple replies or reference the pictures from another website and use the IMG button.
Mike