You may or may not be aware of Google’s new image search feature introduced last year. The feature allows you to use an image as the starting point and find other versions of the image, or even similar images, and the websites they are found on.
One unexpected use of the feature has been to allow owners of images to find unauthorised use of their content on other websites. Are you sure that you have permission to use everything on your site? If not, this article may help clarify.