Ok, first see this:
A fellow blogger friend of mine attempted to ping that trackback URL, but received an error message, which you can see by simply putting that URL in your browser.
He also told me that he WAS able to trackback with the same URL only without the trailing /trackback/ attached.
He emailed me an error message that his MT blog gave him:
You'll have to put that image URL directly into your browser to avoid the hotlink protection.