Mike,
Looks like you may have done it right, but I really can't tell from the URL that you pasted into the "Optional Image" box...
First the image must exist on the Internet. Eg: 
is an existing image on the bertram31.com server, located at http://bertram31.com/bert31_logo-sm.gif
Generally, well almost always, an Internet image is going to end with .jpg or .gif as the file extension. Your rather long URL looks more like where a webpage has the image on it rather than the image itself.
You can find the image URL by right clicking on the image you want to post and select "Properties". The properties popup will contain the full URL. Just high light the full URL, right click on the high light, and select "Copy". The paste that into the "Optional Image" box.
Be sure to Preview you post before actually posting. This will let you know if it worked before it's too late to make changes...
Best regards,
Patrick