Close, but not quite.

1. Click "Share" in the upper left corner of that image's webpage.
2. Copy the "direct link", which looks like this: https://s5.postimg.org/rzbfxkzyf/IMG_0617.jpg
3. Click the little picture of a photo in the upper right of the Sigforum posting window.
4. Paste the previously copied direct link into that "Image URL" box.
5. Click the "OK" button in the image box.


End result: