Just a heads up: https://developers.facebook.com/bugs/361680970914639/
The comment thread on this bug report will be helpful for anyone having trouble right now using ngrok to serve content from their bot in dev mode. Looks like the Messenger Platform blocks ngrok for anything other than pure webhook calls. In other words, if you’re trying to use your bot to serve images in templates, it won’t work. And there’s nothing wrong with your code… it’s the platform.