Many artbooks contains pictures taken directly 
from the anime. Even if they are use on a full 
page or are based on pan-cels, they are not 
hanken.

The differance between a production cel and an 
hanken one is that a production cel was created 
for the anime itself. An hanken is not at all used 
for the anime but only for another produce.
(but that doesn't prevent some production cel to 
be of a quality similar to hanken cel, especialy 
for over-sized one. The differance is really what 
they were created for)

Cheers,
Yann Stettler