Facebook does need to fetch your page before it can show the image in a post. If you just created an article on your site and immediately post it, Facebook won't have fetched it automatically yet. Once it's posted, it will be in Facebook's queue to fetch and update. Usually, that's a fast process, but sometimes, the picture won't show up for hours or even days. We have no control over how long that takes.
With that said, if you use the Facebook URL Debug Tool that you're using above to fetch the page, that will immediately update the cache.
I hope that helps explain, but should you need anything else, just let us know.
Thanks,
Alex