Users can directly modify the .htaccess file in apache
RewriteEngine On
#Replace ?mysite.com/ with your blog url
RewriteCond %{HTTP_REFERER} !^http://(.+.)?mysite.com/ [NC]
RewriteCond %{HTTP_REFERER} !^$
#Replace /images/nohotlink.jpg with your "don't hotlink" image url
RewriteRule .*.(jpe?g|gif|bmp|png)$ /images/nohotlink.jpg [L]
Note: Replace mysite with your own URL and /images/notlink.jpg with your customized anti-theft link statement image.