One effective way to find where your stock images are being used is the TinEye website.

All you have to do is simply upload or link to an image on the internet and Tinagent will search it's catalog of millions of images and show you the results of where they were found.  Although the website / software is still currently in beta (May 2008) it has already catalogued millions of images and provides some very nice results.  This will definatly be something to keep an eye on in the future.