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

What about resizing the pictures to a standard small size and checking for SSIM or luma-only PSNR values? that's what I would do.