In your example, the image is not displayed at all. I suspect this is because you are using postimage.org to host the image, and they block the image request from the external domain (your example). If I substitute the URL of an image hosted on my own server, the background of the image is displayed using the attributes that you set. I would suggest using a different image host.
CSS3 attribute background-size: cover; that you use will scale the image proportionally to fill the browser based on the horizontal width. You do not need to scale the image in advance, although this may not always give you the most beautiful result.
Glenn davidson
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