If you load http://example.com and the HTML there includes an image tag that says '<img src="http://othersite.com/image.jpg">' it means your browser will connect to othersite.com and ask that site for the image. It will not ask example.com.
Doesn't GitHub cache every image in their own CDN for security and control purposes?
But to repeat: auto-updating will not work for Github-hosted content AFAIK.