Don’t you hate it when you put a clear “Do not direct-link” and find out someone has? Here you will learn to avoid that from happening!
On your .htaccess file add this:

RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www.)?domainhere.com/.*$ [NC]
RewriteRule .(gif|jpg|png)$ - [F]

Now, if someone direct-links it will appear as a broken image! But, if that’s not enough and you wnat to be sort-of mean you can use the following code to substitue the image they direct-linked with one you made (like a huge banner).

RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www.)?domainhere.com/.*$ [NC]
RewriteRule .(gif|jpg}png)$ http://www.domainhere.com/image.jpg[R,L]

Note: If you don’t have a .htaccess file go to this tutorial which will explain how to make it: click here.