Well... the background-image property will accept a relative path, an "absolute" aka "root-relative" path, or a URL with equal ease. Internetter's creative terminology causes the most grief.
The relative path in the CSS to the resource should be relative to the CSS file requesting it, exactly as SamA74 stated.
The absolute or root-relative path is relative to the root of the web site.
The URL is a Universal Resource Location that begins with the universal address of the web site and is immediately followed by the root-relative path to the desired resource.