quick Tkinter Label question

[Bill]

I have a label widget with an image being displayed.  The image changes.
Now how do I update the label widget?  assuming I have a ImageTk PhotoImage
called tkim:

root = Tk()
l = Label(root, image=tkim, bd=0)
l.pack()

.... tkim changes...

calling l.pack() doesnt update the label widget with the new image.  neither
does calling l.update().  The Intro to Tkinter doc says "The label widget
uses double buffering, so you can update the contents at any time, without
annoying flicker."  Can someone help me on how to do that?

Thanks in advance,
Bill