Copy and pasting your code (and replacing the image url, obviously) works for me. Are you sure that the url is correct? You can inspect the saves dialogue to check whether the background-image property is being correctly applied. If it’s not, are there typos or other errors in your Stylesheet, either in the relevant code or somewhere before it?
While delimiting quotations are only really needed if the URL passed to the url() token contains SPACE (or similar) characters, I always delimiter my URLs just to be safe…