Ouvrir une image au format pgm ASCII

def ouvrir_image():
    global photo
    image=filedialog.askopenfilename() 
    photo=PhotoImage(file=image)
    largeur=photo.width();hauteur=photo.height()
    fen.geometry(str(largeur+2)+'x'+str(hauteur+2))
    fen.title(image)
    can.configure(width=int(largeur+2), height=int(hauteur+2))
    can.create_image(largeur/2+1,hauteur/2+1,image=photo)
    can.pack()

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Python32\lib\tkinter\__init__.py", line 1399, in __call__
    return self.func(*args)
  File "C:\Users\DezertPunk\Desktop\test - Copie.py", line 27, in ouvrir_image
    photo=PhotoImage(file=image)
  File "C:\Python32\lib\tkinter\__init__.py", line 3228, in __init__
    Image.__init__(self, 'photo', name, cnf, master, **kw)
  File "C:\Python32\lib\tkinter\__init__.py", line 3184, in __init__
    self.tk.call(('image', 'create', imgtype, name,) + options)
_tkinter.TclError: couldn't recognize data in image file "C:/Users/DezertPunk/Desktop/hep.pgm"