When you hit "post reply" to a thread (or to start a new topic) underneath the window you type in is another set of boxes - the heading is Add an Attachment.
If you click the browse button to the right of the first window below the big type your post in box, it will look at your computer. Navigate to the picture on your computer. From there to the right of the box below the browse box click "add attachment" and it will attach the pic to the post. Click Submit and it's there.
If it is from an external link click the button above the main text box marked Img and paste in the link there. Then click close tags and it will add it to the post.