Here's what their compete chart looks like, for what little it's worth (login required, so screenshot instead): http://cl.ly/image/3z1v152G3r1l
For images it should be hidden.
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.