You *can* make blank placeholders. But they have to be accessible on a web server, "/etc/squid/zaps" is not a web URL. Turn apache on on the machine you have Squid installed on and stick your images in "/var/www/html/zaps" and use "http://yoursquidserver/zaps" as the URL.
Also, I believe you could have the zapper script just strip all occurances of the ad but I see two problems with this. One is it would screw up the formatting of the web page and the second is you would have to heavily modify your "squid_redirect" script (if that would be enough to do it). It wouldn't hurt for you to open that script up and take a look at it. You should see things like where the environment variables are set to the default URL of http://adzapper.sourceforge.net/zaps.



