Interestingly, that link shows a twitter card with the image in it for me. I assume it's cached on Twitter's side as the image URL is this: https://o.twimg.com/2/proxy.jpg?t=HBgoaHR0cHM6Ly90d2l0cGljLm...
"t" is a Base64-encoded blob of data containing a few miscellanous bytes, as well as the URL the proxy should download (in this case, twitpic.com/show/large/...). "s" is the signature, used so we can't just abuse the proxy to download more.