That assumes all thumbnails or pictures hash to the same value. Why would that be? There can be different algorithms, resolutions, crops... And there can be millions of thumbnails to check.
The same set of values, yes.
> Why would that be? There can be different algorithms, resolutions, crops...
Yes, so you do normalization and reduction before computing the hashes.
And ‘hashes’ doesn’t have to mean literal SHA. Any non-reversible fingerprinting algorithm will do.
the internet definitely remembers forever, unfortunately, especially porn