Slack API Attachments: image_url containing https

I am using Slack's rtm api and the following works just fine:

slack_client.api_call(
        "chat.postMessage",
        channel=channel,
        text='Some text',
        attachments=[{'fallback': 'This is fallback text',
                      'title': 'This is a title',
                      'text': '',
                      'image_url': 'http://<path>image.jpg',
                      }]
    )

However, if I change the image_url to a jpg image that happens to be stored at an https address, the image doesn't appear. Is there a general issue with using https addresses? Is there a work-around?

I've tried other formats (jpg and png) and other files. The url I am using opens up fine in a browser, so there is no problem with the url link itself.

The https site itself happens to be a Dropbox folder, but the link itself is to a public file.