I'm working on a tool to help people out with this problem. My tool will monitor the internet and email you when your images are used on other sites. For a current solution, you'll need to do a google reverse image search, one by one for each image, periodically. This is tedious work nobody really wants to do. My tool will be making this process a whole lot easier. All you have to do is type in your site address, the tool will scan your site for all images and will email you a report every week on where your images are being used.
To join as a beta tester or early adopter, please submit your email on my landing page www.checkmypics.com .
I'm also looking for other techies to help out with development. Experience with scraping is a plus!