it seems that Bitnami really use PIL the original library, not the Plone fork Pillow.
I was not even imagining that Plone could use it.
See
the 1.1.7 is the most 'recent' (less ancient) version to download.
A vanilla 5.07 install on my computer has picked the 3.3 version of Pillow, a 5.1 Plone Pillow has picked 4.2.
It would be interesting to see what the buildout from Bitnami contains.