g1smd - 8:47 pm on Nov 7, 2012 (gmt 0)
Once the image has been requested once, it's then cached in your browser and irrespective of what condition does or does not match in the htaccess file next time, if you change your IP or change the conditions in the htaccess file, your next request will continue to show the image cached in your browser. This makes it look like the htaccess rules are failing.
This means that the rules will not work if the user has previously seen the image loaded in a legitimite way. You'll need to set the image properties (both the real and the replacement) so that it is not cached - because to the user it's always the same URL and browsers only know URLs and not anything about the server internals.