TinEye is a reverse image search engine that search visually similar images with an image. Unlike other search engines which retrieve image results on the basis of keywords and meta description, Tineye devised a technology using crawlers and advanced image recognition software to look for visually similar images around the web.
TinEye not only find the exact matches of images but also find those which are modified or edited. TinEye crawler search and compare an image passed as a query with over billions of images indexed in their database. If the image you are searching for not found on their index then you can add the image to the TinEye database. To submit an image you need to create free account because only the register members can submit new images to their database and the images submitted by unregistered users are automatically discarded after 72 hours unless a register user save the same image.
So, why should you use TinEye, here are the best reasons: a) Find sources of images – It allows you to find where the images came means on which webpages the images are attached. b) Find Modified or Altered Copies - To discover modified or edited versions of an image (*impressive feature). You can find images of different sizes and resolution. 
c) To combat Image plagiarism – particularly helpful for webmasters. It help you to make sure that your original content is safe & not plagiarized. d) Find Social Profiles - an interesting possibility of Tineye. TinEye can be use to find people via profile pages created on social networking websites. See example.
There are actually three possible ways to add an image as query to TinEye : a) Upload from computer b) Copy and paste image URL to TinEye Search box. c) There are addons and extension for Firefox and Chrome browsers to make search more easier for you. Go to plug-in page.