Tinyeye

TinEye, according to developers' assurances, searches for pictures not by tags and name, but by an already existing picture. That is, having any image you can try to find it in the best quality, find something similar or just understand where it comes from. While the search engine is in the state of closed beta testing, you can subscribe to an invite on its page.

A similar service was supposed to appear, but I did not think that it would happen so soon.