title: _tkinter.TclError: bitmap "pyimage2" not defined
body: <p>I want to display an icon on a menu bar, so using this <a href="http://effbot.org/tkinterbook/menu.htm" rel="nofollow">information</a>, I coded this:</p>
  
  <pre><code>img = Image.open("help.png") 
  menubar.add_cascade(label="Help",menu=helpmenu,bitmap=ImageTk.PhotoImage(img))
  </code></pre>
  
  <p>I got this error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "mine.py", line 67, in &lt;module&gt;
      m.menus(root)
    File "mine.py", line 55, in menus
       menubar.add_cascade(label="Help",menu=helpmenu,bitmap=ImageTk.PhotoImage(img))
    File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 2699, in add_cascade
      self.add('cascade', cnf or kw)
    File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 2696, in add
      self._options(cnf, kw))
  _tkinter.TclError: bitmap "pyimage2" not defined
  </code></pre>
  
  <p>How to fix this ?</p>