Yeah, that's not an image URL, it's a URL to a script. This is disabled in threads, because allowing users to embed a script in the page poses a pretty big security risk.
Actually this site was hacked this week due to a similar exploit in another software program installed (not threads). You could modify things to allow it, but I sure wouldn't. If you take the valid image checks away, the user could potentially execute code and/or read cookies of your users using the image tag.
In the past Classic (and actually in the past UBB.Threads) didn't have the checks in place to make sure it's really an image URL being used.