如何转型图片对象到缓冲区字符串？

Question

sbuf = StringIO()
sbuf.write(decoded_file)
pimg = Image.open(sbuf)

print pimg.size

cimage = StringIO()
pimg.seek(0)
pimg.save(cimage,'jpeg')
contents = cimage.getvalue()
if contents == decoded_file:
   print '1------->> ok!'

“decoded_file”是图像缓冲区字符串。我要转换图片对象“pimg”到“contents”和“contents”应等于“decoded_file”。在