The Rembrandt Panel in ETH Oberon
The tutorial was written by André Fischer and Hannes Marais. The <a href="https://en.wikibooks.org/wiki/Oberon/hall" class="extiw" title="b:Oberon/hall">Hall of Fame</a> attributes Gadgets to Hannes Marais (jm) and Patrick Saladin. Therefore the illustration was probably drawn by André or Hannes (Johannes) or Patrick.
CC BY-SA 4.0
In ETH Oberon the image is used in the Web based tutorial entitled "Rembrandt User's Guide".