403 error for loading image from http and not https

My question is similar to this one, but I'm using html & javascript only.

If I try to load this image over https it shows the image just fine.

<img src="http://ift.tt/1En6NZh">

Except for this warning

Mixed Content: The page at '' was loaded over HTTPS, but requested an insecure image 'http://ift.tt/1En6NZh'. This content should also be served over HTTPS.

But over http it produces this 403 error.

GET http://ift.tt/1En6NZh 403 (Forbidden)

I don't understand why because the image url is http.

Any idea what to do?

Many thanks !