How did you post it? Did you just include the hyperlink using the “from the web” option in the image uploader?
I assume it is done so that we won’t run into situations where the link is broken due to someone elses’s actions. On the other hand, I had that image on my desktop (the kids had downloaded it to try to convince my wife and I that we’d be so much better off with a tea cup pig as a pet…) so when I attached it, it was already uploaded to the forums, directly.