Images pages:

http://www.med.yale.edu/library/historical/fuchs/

It might be worth their while to pick up the Dover book  Ready-To-Use Medieval Herb, Plant and Flower Illustrations: 294 Different Copyright-Free Designs Printed One Side (Clip Art Series) and scan them.  Any one can use them for free.

[non-period]
Color illustrations here: http://www.canoe.ca/HealthHerbal/home.html