..image tracker? Those are legal?
Very much so, I have used quite a few when trying to remember what/where I got a picture of various pieces of building photos (Like this cool one that has a door and a rail-less staircase coming out of a sky scraper and back into a lower door).
But yes, you upload a photo and it runs a standard search trying to find a matching picture, and checks pixels etc in case the image was resized. Very easy program to write as all it does is compare, just requires a decently powerful server to run due to the bajillion bots involved.
There are many on the internet, though it is hard to find a free one (I dunno, the last time I used one was a year ago).
