A WebmasterWorld thread discusses a not-so-talked-about feature about how to locate whether your images have been used (or stolen) on your domain. Ogletree writes:
I just found out you can do a site:domain.com search in Google images and get a list of images on a site. It will also show images that other sites are stealing.
This is actually covered in the Google Image Search help page:
Can I do complex searches with image search using Boolean operators and other Google commands?You can use all operators for image search that you would use in a Google text search. For example, you can use "site:" with image search to restrict your search to images on a particular website. So, to find all the pictures of lemurs at the Duke Primate Center, the query would be " site:duke.edu lemurs". You can also refine your search using the Advanced Image Search page.
Here's an example of the site operator in action. Note how you will find results from sites who also link to your images.
Forum discussion continues at WebmasterWorld.