I don't know if the reason is due to the node images being createWindow canvases themselves but that doesn't seem like it should matter

Probably. Look at the source http://search.cpan.org/grep?cpanid=SREZIC&release=Tk-804.030&string=createWindow&i=1&n=1&C=0, createWindow is hardly tested, and is never tested with canvas/within/canvas