Something like this would work:
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^http://(www\.)?yourdomain.com(/)?.*$ [NC]
RewriteRule .*\.(gif|jpg|jpeg|bmp)$
http://www.yourdomain.com/hotlink.jpg [R,NC]
Basically, if any domain other than yours attempts to display an image, it'll be replaced with hotlink.jpg (which can be a ******* pic, for example).