To display an picture that's on the web, you have to put the link in an image tag. After clicking reply, look for the IMG button. Click that and insert the link into the pop up box.
I tried your link but there must be something wrong as it ends up going to a main page and not a picture. Perhaps that site does not allow off site posting?
If so, the only other choice is to save the pic to your computer and attach it as a file to the post. That option is at the bottom of the reply page.
Checking the FAQ page will probably help explain this a bit better than I can.