Some values can be files, especially images.

Syntax :

url(http://www.my-web.com/file.html) /*absolute address */
url(../extra/image.gif) /* local image (from the "extra" source directory) */
url(../wit-icons/image.gif) /* local image, already used as an illustration by at least one idea. */

Note : Urls to images used by WIT must be those which will be effective when the site is produced. So you MUST use relative URLs. Especially, all image URLs are relative and must start by "../" and related image files will be placed in the "extra" sub directory of the current WIT document save directory.