python-tk: tkinter photo doesn't work inside a function

Wow, python-tk sucks. When I create the photo inside a function, it gets garbage-collected because it's not smart enough to know that the label is using it. So I have to manually keep it from being deallocated (in this case, by making it global). Also, I'd really like to use after_idle instead of after, but it never gets around to actually updating the display.

#!/usr/bin/python

import Tkinter
import ImageTk,Image

root = Tkinter.Tk()
root.geometry('+640+480')

old_label = None
label = None
photo = None
def display_image(root, image):
  global old_label
  global label
  global photo

  root.geometry('%dx%d' % (image.size[0], image.size[1]))

  photo = ImageTk.PhotoImage(image)

  label = Tkinter.Label(root, image=photo)
  label.place(x=0,y=0,width=image.size[0],height=image.size[1])

  if old_label is not None:
    old_label.destroy()

  old_label = label

def image_loop():
  global root
  print '.'
  image = get_image()
  display_image(root, image)
  root.after(1000,image_loop)

image_loop()

root.mainloop()