To send an image to specific domains:
RewriteEngine on
RewriteCond %{HTTP_REFERER} .*fusker*.*$ [NC]
RewriteCond %{HTTP_REFERER} .*usefulidiot*.*$ [NC]
RewriteRule .*\.(gif|jpg|jpeg|bmp|png)$ http://www.primanudes.com/hotlink/hotlink.jpg [R]
This says if one of those specific domains asks for an image, send them something else.
To send an image to all hotlinkers:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.primanudes\.com
RewriteCond %{HTTP_HOST} !^primanudes\.com
RewriteRule .*\.(gif|jpg|jpeg|bmp|png)$ http://www.primanudes.com/hotlink/hotlink.jpg [R]
put YOUR domain in the second line. If your site resolves with other names, add those as well (either on their own lines, or, use a regEx to do it). That says if it's not your domain asking for an image, send them something else.
If you have a bunch of subdomains, use something like:
RewriteCond %{HTTP_HOST} !^.*primanudes.com.*
Here's a thread with more about .htaccess:
http://www.gofuckyourself.com/showthread.php?t=413223