Pythonコードのデバッグ問題：IOError: イメージファイルを識別できない

# -*- coding: utf-8 -*-
'''
Question 0005: You have a directory with a lot of photos, make them all no larger than the size of the iPhone 5 resolution
'''

import os
import Image

def changeSize(path):
	for picName in os.listdir(path):
		picPath = os.path.join(path, picName)
		im = Image.open(picPath) 
		w, h = im.size
		im.thumbnail((1136, 640))
		im.save('finish_'+picName.split('.') [0]+'.jpg','jpeg')

if __name__ == "__main__":
	changeSize(r'D:\notes\python26Q\CR7')