possibly the reason the htaccess fixes aren't working is because the images are being stolen from your client's sites, not yours. however, are your clients using an extended version of your domain (i.e.
http://www.yoursite.com/hosted/theirsite), or do they have their own domain (i.e.
http://www.theirsite.com)? if it's their own domain, that's probably why the htaccess files aren't working, but if it's an extended version of yours, i can't really help. just to check, you should try using the php image trick, and move the shared folder so that the stealer will lose the links altogether. maybe then you could apply another htaccess file, also.