www: Fix background sprint path

www/source/stylesheets/all.css.sass

@@ -31,7 +31,7 @@ $color-complement-3: #00822C
 $color-complement-4: #00501B
 
 body
-  background-image: url("/images/escheresque_ste.png")
+  background-image: image-url("images/escheresque_ste.png")
 
 nav.navbar.navbar-static-top
   border-radius: 5px