It looks to me like you were pretty close, but the form of the URL you included was a web page (using PHP) instead of a direct link to an image file.The web page URL is:
http://img210.exs.cx/my.php?loc=img210&image=fostex166ehorn51ae.jpg
This can be put in the "Optional Link URL" field.
The image file URL is:
http://img210.exs.cx/img210/9044/fostex166ehorn51ae.jpg
You can find out the direct link to an image file URL by right clicking on the image, and then clicking "Properties." Try it!
This can be put in the "Optional Image URL" field. It will show the picture below at the top of the post.
Cool speaker, by the way!