For example: <img src=http://www.ezboard.com/image/logo.gif>
NOTE: 'http://www.ezboard.com/image/logo.gif' is where you would post the URL of the image you want to post.
Then make sure the image is online and downloadable and that in the 'Posting Options' section, under the 'Your Comment' box, you tick the 'HTML...' box.