TinEye is a reverse image search engine. You can upload an image from your computer and search the Internet for it. You can also paste a image URL and find other places that the image exist on the Internet. What is neat is that you don't necessarily have to have the same file name. TinEye will use Image Identification Technology, not keywords, metadata, or watermarks. The site currently has 1.6 billion images indexed for you to use in your search.
TinEye would be a great site to use to check on students using images in a presentation but did not site the source for where they received the image. It will also help with those teachers who have students publishing websites to ensure that students did not use copyrighted photos.