title: What algorithm could be used to identify if images are the "same" or similar, regardless of size?
body: <p><a href="http://tineye.com/" rel="noreferrer">TinEye</a>, the "reverse image search engine", allows you to upload/link to an image and it is able to search through the billion images it has crawled and it will return links to images it has found that are the same image.</p>
  
  <p>However, it isn't a naive checksum or anything related to that.  It is often able to find both images of a higher resolution and lower resolution and larger and smaller size than the original image you supply.  This is a good use for the service because I often find an image and want the highest resolution version of it possible. </p>
  
  <p>Not only that, but I've had it find images of the same image set, where the people in the image are in a different position but the background largely stays the same.  </p>
  
  <p>What type of algorithm could <a href="http://tineye.com/" rel="noreferrer">TinEye</a> be using that would allow it to compare an image with others of various sizes and compression ratios and yet still accurately figure out that they are the "same" image or set? </p>