Newbie Tkinter Question

[Pseud O'Nym]

Hi,

the following fragment doesn't cause any errors and results in buttons the
correct size for the images, but the buttons don't work, and the images
aren't displayed. If I add a text property, and remove the images, they work
fine, so the rest of my code's OK. I've searched this group and Python.org
to no avail.

class App:
  def __init__(self, master):
    frame = Frame(master)
    frame.pack(side=LEFT, fill=Y)
    image1 = PhotoImage(file='button_a.gif')
    self.button = Button(frame, image=image1)
    self.button.pack(side=TOP)

Can anyone enlighten me?