Here is the same question answerd https://stackoverflow.com/questions/940451/using-relative-url-in-css-file-what-location-is-it-relative-to.
here is the W3C spec from the answer http://www.w3.org/TR/REC-CSS1/#url
With other words it depences on the Folder Structur, as commented under your Post, by Sport Billy.
i hope it helps