Rhino knows about no image editor, it calls just open on the image file and Windows handles the rest. See Bitmap editor from within Rhino, and more specifically Bitmap editor from within Rhino - #9 by jesterking. But then again, you already know about the thread Bitmap editor from within Rhino - #15 by rabbit
Anyway, I logged RH-65337 Make image editor to use configurable in Rhino.