If you can import the image into Dokuwiki namespace using the media manager, then you can link with the {{}} syntax, see
https://www.dokuwiki.org/images
Using [[]] will indicates an hypertext link, either external (use [[http:// ...), internal ([[<namespace:><page>]]) or interwiki ([[wiki>page]]).
If you are trying to link to a file using an absolute path, then remember that Apache has to serve the file to Dokuwiki, so Apache's restrictions come into play. Files must be visible under DOCUMENTROOT, or aliased, or explicitly set up in some other way. Apache will refuse to serve absolute paths, would you really like a cracker downloading
/etc/passwd for instance?