PIL:无法保存用png粘贴的jpg

#!/usr/bin/env python3

from PIL import Image
from PIL import ImageDraw

im = Image.open("existing.jpg")
logo = Image.open("python-32.png")

back = Image.new('RGBA', im.size)
back.paste(im)
poly = Image.new('RGBA', (512,512))
pdraw = ImageDraw.Draw(poly)
pdraw.polygon([(128,128),(384,384),(128,384),(384,128)],
          fill=(255,255,255,127),outline=(255,255,255,255))

back.paste(poly, (0,0), mask=poly)
back.paste(logo, (im.size[0]-logo.size[0], im.size[1]-logo.size[1]), mask=logo)

back.show()

当我执行上面的代码时,我可以看到一个PNG图像显示为一个随机名称,如 tmpc8rb455z.PNG

我还尝试用jpg格式保存它,但失败了。意思是当我添加 back.save('res.jpg', 'JPEG') 然后执行它,我得到这样一个错误:

Traceback (most recent call last):
  File "test.py", line 32, in <module>
    back.save('res.jpg', 'JPEG')
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/PIL/Image.py", line 1893, in save
    save_handler(self, fp, filename)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/PIL/JpegImagePlugin.py", line 604, in _save
    raise IOError("cannot write mode %s as JPEG" % im.mode)
OSError: cannot write mode RGBA as JPEG

back.save('res.png')

它可以工作,但大小 res.png 大于5倍 existing.jpg