title: Python Tkinter update image
body: <p>I have problem with updating picture in canvas in tkinter. I'm using Python 3.3.</p>
  
  <pre><code>from tkinter import *
  
  class Testi():
      def __init__(self):
          self.canvas = Canvas(root, width = 800, height = 480)
          self.img = PhotoImage(file="title.pgm")
          self.imgArea = self.canvas.create_image(0, 0, anchor = NW, image = self.img)
          self.canvas.pack()
          self.but1 = Button(root, text="press me", command=lambda: self.changeImg())
          self.but1.place(x=10, y=500)
  
      def changeImg(self):
          newimg = PhotoImage(file="cell.pgm")
          self.canvas.itemconfig(self.imgArea, image = newimg)
  
  root = Tk()
  root.geometry("800x600")
  app = Testi()
  root.mainloop() 
  </code></pre>
  
  <p>At first the image is visible but after pressing button, it goes blank. I have read many similar questions but none of those had any help.</p>