where does WP put in it’s image links?

Ok – so after some hair brained adventures in doing preg_match and str replaces on the the image src (wanted to run it through some gd stuff before it got to the page)… i now have a new problem. It turns out WP is using a preg_match of their own to create the <a href link that equals the img src …. anyone know which file this match (or replace) is happening because if it’s going to add links they need to be to the full size image not to the newly created thumbnail.