The new reference.pnm is a freely licensed replacement. The photo has
been taken by Reinhard Tartler on August 28 2014, and is licensed under
the expat license as stated at http://www.jclark.com/xml/copying.txt

(cherry picked from commit 8895bf7b)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>