Hi there,
Recently I had a problem with displaying images and solved the problem itself.
To display images on preview AND device (as release or run --prod) use should use the location “…/assets/” not “assets/” or “…/…/assets”.Other pathes are only working on debug live preview.
Could you please just let your post be your post if you solve it? It could also benefit others who are searching the forum.
Done! 
dont work for me.
i try background-image: (’…/assets/img/auth-background.jpg’) no-repeat center center fixed;
AND
background-image: (‘assets/img/auth-background.jpg’) no-repeat center center fixed;
but ONLY device not display