Oh boy. Everyone who has uploaded images of screenshots of illegal movies onto forums are gonna be majorly shafted now.
Given the sheer number of variables involved in the encoding process, the odds of independent encodes being absolutely identical are extremely low.
It is trivial to determine from a screenshot whether it is taken from the source or a re-encode (i.e. whether it is from a legitimate source or not, assuming the source is the only authorized copy), and if you have a copy of the particular re-encode, to verify the exact provenance.