Hotlinking is discouraged, and that's an example of why: some servers can detect the hotlink and refuse to send the image. It may appear okay for you because you already have the image in your browser cache, but it's broken for us.
(There's also the moral question about stealing other people's bandwidth, but there are differing opinions on that.)
Re-upload images to a service like imgur.com or imageshack.us. It's very easy to do by URL only.