“Image Hotlinking” is when another website embeds an image from your site (usually without permission). This increases your bandwidth bill, is a potential security concern, and is generally a pretty rude thing to do. To prevent this in Drupal, add the code below to your sites/all/files/.htaccess
file:
# image hotlink protection
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{REQUEST_FILENAME} -f
RewriteCond %{REQUEST_FILENAME} \.(gif|jpe?g?|png)$ [NC]
RewriteCond %{HTTP_REFERER} !^https?://([^.]+\.)?domain\. [NC]
RewriteRule \.(gif|jpe?g?|png)$ - [F,NC,L]
</ifModule>