1. 3.3 is in security fixes only mode and shutdowning mechanism was changed in 
3.4.

2. Looks as some Tk root was not explicitly destroyed and deleted during 
shutdown stage.

3. Tk.destroy does nothing with images, it destroys only subcomponents and 
commands. In any case it destroys only Tcl/Tk objects, not Python objects 
which wrap them.

4. I think we shouldn't use _default_root outside the tkinter package. This is 
implementation detail.

5. Agree, we could patch Variable/Image.__del__, and proposed patch looks 
correct (other way is pass TclError as default value for optional parameter: 
"def __del__(self, TclError=TclError)", both ways are widely used). But I 
prefer first try to reproduce this in tests.