Does htaccess RewriteCond affect performance?

Hi, Performance issues aren't really my forte:

background info:
My test site is geared to anonymous only, and thus have Boost. Each node has a cck field so that users can type in a word eg. "DOG". I've hotlinked an image url to automatically include that cck word - eg. http://mysite/images/DOG.jpg. This allows me the choice to easily upload a picture or not. Obviously a 'broken image' icon appears if no pic is uploaded.

To resolve the 'broken link' ugliness I've come across something like:

RewriteCond %{REQUEST_FILENAME} !-f

RewriteRule ^images/.*.jpg$ /images/blank.jpg [L]

The above will enable me to substitute the 'broken image' icon with an image of my own.

My questions:
1) will the Rewrite conditions have significant performance impacts if there are 15 thousand (or more!) missing images?
2) will the Rewrite conditions affect Boost or any other caching systems?

thanks!