Possibly by it being linked from a torrent description saying "Here is this movie that I pirated and am offering to you illegally for free - <username>" and then having that same username attached to the ImageShack screenshot that also provides your IP address?
Plus, if we're being pedantic, most pirated movies shared online are re-encodes of their original source material.
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.