[anarchystar]

if you have .htaccess file control you can put the following lines in it:

RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?yourdomain.com/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://(forums\.)?yourdomain.com/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://(www\.)?google\. [NC] 
RewriteCond %{HTTP_REFERER} !^http://(www\.)?search\?q=cache [NC]
RewriteCond %{HTTP_REFERER} !^http://(www\.)?yahoo\. [NC]
RewriteRule \.(gif|jpg)$ - [F]

This will allow only the domains listed to link to images on your site, notice that I have the line:

RewriteCond %{HTTP_REFERER} !^http://(forums\.)?yourdomain.com/.*$ [NC]

that is for the subdomain forums.yourdomain.com ... any subdomain you want to allow access to imgages must be listed as such also

I also allow search engine domains to link to my images fro cacheing of pages, but you don't have to. If you don't want search engines to cache pages, just remove those lines

and finally, the line:

RewriteRule \.(gif|jpg)$ - [F]

tells the script what kind of images to apply the rules to. you can also add .bmp's or .png's by adding a pipe and the new file extenstion like this:

RewriteRule \.(gif|jpg|png|bmp)$ - [F]