Mod Rewrite Question

My question is actually rather simple and relates to mod_rewrite (one of the most useful but most poorly documented features of apache).

Basically, I am using standard rewrite code in the hopes of making any image when called on its own (not inside a page) display my rewrite rule (a special page).

So far the best I've been able to do is find the Rewrite Condition
RewriteCond %{REQUEST_FILENAME} which displays data about the requested file name (I had tried REQUEST_URI but every request falls subject to that even when called within an image).

As far as I can tell, REQUEST_FILENAME should do the trick, but I can not for the life of me figure out the proper syntax to get it working or even tell if it will do what I need.

So, if you have any ideas or tips for how to use mod rewrite to take an image called separate from any file and send it to a rewrite rule, please help. Below is the code I am working with:

[code]
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://www.extremefactor.com.com/.*$ [NC]
RewriteCond %{REQUEST_FILENAME} ????
RewriteRule .*\.(gif|GIF|jpg|JPG)$ http://