What algorithm could be used to identify if images are the “same” or similar, regardless of size?

Question

TinEye, 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 link

Answer 1

Check out this blog post (not mine) for a very understandable description of a very understandable algorithm which seems to get good results for how simple it is. It basically partitions the respective pictures into a very coarse grid, sorts the grid by red:blue and green:blue ratios, and checks whether the sorts were the same. This naturally works for color images only.

The pros most likely get better results using vastly more advanced algorithms. As mentioned in the comments on that blog, a leading approach seems to be wavelets.