see
http://www.amateurlodge.com/hotstop.txt
is what I use, I hotlink my pics across 3 domains.
put in in your root dir, but if you use picposts you'll have to add them to it as well.
the best way it to create a content folder that only you are allowed to hotlink from and put your htaccess in that folder, then it's easy to serve pics and banners from other dirs when you need to.
Hope it helps.
J