When you write your post or start your thread, make sure you are using the advanced frame. In your control panel you need to go to Edit Options and ensure that 'Enhanced Interface - Full WYSIWYG Editing' is selected under Message Editor Interface. Once that is on you can write your post using the advanced options. In your post, hit the 'Insert Image' icon (little yellow box with mountains drawn on it) that will call up a dialog box that you can insert the url from your picture into. When you hit OK it will automatically link the image into your post. You can't 'post up' pictures from your hard drive, you can only link to them, so you need separate image hosting to do that.
HTH.