Tkinter displaying an image in a button

[David R. Smith]

The first code snippet below successfully displays an image, whereas the
second just shows gray where the image should be.  What's wrong with the
class version, and what do I have to do to get it to work? 
(I am using Python 2.0.1 on Debian.)

    adTHANKSvance
    David Smith



from Tkinter import *
import Image, ImageTk
root = Tk()
img = Image.open('SonicCruiser.jpg')
phi = ImageTk.PhotoImage(img)
button = Button(root, image=phi)
button.pack()
root.mainloop()



from Tkinter import *
import Image, ImageTk
class App:
    def __init__(self, master):
        img = Image.open('SonicCruiser.jpg')
        phi = ImageTk.PhotoImage(img)
        button = Button( master, image=phi )
        button.pack()
root = Tk()
app = App(root)
root.mainloop()