I see quite a lot of misconfigured sites that look to use various methods to block ‘hot’ linking of images, but return links to such images in the embedding data e.g.
Checking with http://debug.iframely.com/ I really think this isn’t a Discourse problem per se, but it does look poor.
One thought is that the onebox creation could retrieve the image when cooking the post and either store a thumbnail to serve later, or if it couldn’t be retrieved act as though there was no image specified.
Storing a copy of the image would be fairly robust and future-proof, and I think should fall under fair-use for copyright purposes (the original site isn’t going to be worse-off for the reuse of a 130x90 thumbnail which was retrieved from meta-data, though IANAL).
Failing that I tried making a component to catch the error event on images and then add a class to display:none them or their wrapper. I’ve not succeeded yet as I got bogged down in decorateCookedElement()… I’m not sure if I’m looking in the right place.
So, is it just me that seems to suffer broken images frequently in oneboxes - has anyone else got a workaround?