There is no real good way to change what editor is used on the edit command for images. This is by default for Windows set to MS Paint.
I have used https://defaultprogramseditor.com/ with success to change what editor is used when using the edit command (right-click on image in windows explorer, choose edit from the context menu). I have my Windows setup to use GIMP for that edit, so my Rhino opens images in GIMP when clicking the pencil.
It would probably be nicer if one could just set somewhere in Rhino what program to use.
@ivelin.peychev if I am not mistaken the advanced setting Toolbars.BitmapEditor is for editing the icon images of toolbars.