When you're posting a reply, you have two ways of doing it. You can press the "Host an image" button and upload it there on servimg, or find another website that hosts images (sadpanda.us, imageshack.us, etc). When you upload an image, you'll get a link to it... if it's a bare link (http://website.com/images/1204912.jpg), use the Insert Image button and paste the link there. However, some websites create ready-to-use codes for clickable image thumbnails.