I'm surprised the test forum allows you to attach a picture from your hard drive. (I just navigated over there and it allows me too). When you navigate to the general forum that option is taken away.
There has been talk that the TW website only allows posting of pictures that are already posted on the web somewhere else (a different forum, snapfish, kodak gallery, photobucket, etc). This was a way to save server space and keep the costs down. That is the reason the general forum is asking for the URL, the URL is the web address of the photo you are wanting to post. It does not give you the option to browse your hard drive to find a picture saved on your computer (even though this option is available in the test forum). When you browse your hard drive and attach a picture, the picture is saved on the TW website server. When many people start adding pictures this way the server starts filling up, and it costs more money for more server space.
If the server space/cost is truly the issue, I wonder how long it'll be before the test forum no longer allows you to attach pictures. I don't believe it was their intention to allow you to attach pictures in the test forum. Otherwise, everyone will post their pictures on the test forum then link the URL to the general forum.
Here's an example, I entered the URL from the picture you attached in the test forum:
If you go to this webpage, http://www.topspeed.com/motorcycles/...ure194541.html, then go to an image and right-click the mouse and select "copy image URL" then paste that URL into the attachment section it will display the photo on the TW site: