Indicate a little more context (e.g. class definition, import).
Remember to add an object imageto your window (before displaying the image and window):
self.window.add(image)
button, :
image = gtk.Image()
image.set_from_pixmap(pixmap, mask)
image.show()
button = gtk.Button()
button.add(image)
window.add(button)
button.show()
button.connect("clicked", self.button_clicked)