The technology behind the World Wide Web, the Hypertext Transfer Protocol (HTTP), does not make any distinction of types of links — all links are functionally equal. Resources may be located on any server at any location. Linking to an image stored on another site increases the bandwidth use of that site even though the site is not being viewed as intended. The complaint may be the loss of ad revenue or changing the perceived meaning through an unapproved context.

Here is easy and simple way to stop hotlinking :
Open lighttpd.conf file:
$ vi lighttpd.conf

#### stop image hijacking (anti-hotlinking)
$HTTP["referer"] =~ ".*BADDOMIN\.com.*" {
        url.access-deny = ( "" )
#      url.access-deny = ( "jpg", "png", "js", "jpeg", "gif" )
}

You can also enforce password protection:

$HTTP["referer"] =~ ".*BADDOMIN\.com.*" {
 auth.require = ( "/" =>
                   (        "method"  => "digest",
                            "realm"   => "Authorized users only",
                            "require" => "valid-user"
                   )
                 )
}

Restart lighttpd:
# service lighttpd restart