Here's the section from my .htaccess. Put it in the root of your site, and it should stop anyone from linking to any file on your site.
You'll see that I have several domains pointed to my site, so I have those listed so that they will be able to show the images while browsing the site under any of those domain names.
I also have to list the name of the "alternate" image, so that it can be displayed.
Your mileage will vary, but this should give you a start...
Code:
Content visible to verified customers only.
If you try to link to any image on my site, you should get:
Code:
Content visible to verified customers only.