As below, I want to show an image in canvas using tkinter, and when a click the button, an other photo should be shown. But I failed. The first image shows well, but the image didn't change when I click the button

```
C = Tkinter.Canvas(top, bg="#fff", height=500, width=600)

// show image1 in canvas first and it works
itk = ImageTk.PhotoImage(img1)
C.create_image(300, 250, image=itk)
C.pack()

def changeImage():
    // I want to show image2 in canvas, but I fails
    print 'change image in canvas'
    itk2 = ImageTk.PhotoImage(img2)
    C.create_image(300, 250, image=itk2)

button = Tkinter.Button(top,text='click', command=changeImage)
button.pack()

top.mainloop()

```