title: TclError: couldn't recognize data in image file "background.png"
body: <p>I've switched over to Python 2.7 (from 3.4) recently, and I'm having trouble reading a .png file with Tkinter Photoimage class. I say:</p>
  
  <pre><code>background = "background.png"
  photo = tk.PhotoImage(file=background)
  </code></pre>
  
  <p>But I get this error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "main.py", line 36, in &lt;module&gt;
      photo = tk.PhotoImage(file=background)
    File "c:\Python27\lib\lib-tk\Tkinter.py", line 3323, in __init__
      Image.__init__(self, 'photo', name, cnf, master, **kw)
    File "c:\Python27\lib\lib-tk\Tkinter.py", line 3279, in __init__
      self.tk.call(('image', 'create', imgtype, name,) + options)
  _tkinter.TclError: couldn't recognize data in image file "background.png"
  </code></pre>