Windows下正确运行代码如下：

from PIL import Image

# 打开一个jpg图像文件，注意是当前路径：

im = Image.open('easy2.jpg')

# 获得图像尺寸

w, h = im.size

print('Original image size: %sx%s' % (w, h))

# 缩放到50%

im.thumbnail((w//2, h//2))

print('Resize image to: %sx%s' % (w//2, h//2))

# 把缩放后的图像用jpeg格式保存

im = im.convert('RGB')

im.save('thumbnail.jpeg') 

关于原示例代码报错如下：

"D:\Program Files\python-3.7.9\python.exe" D:/py/test-three/test_01.py

Original image size: 680x154

Resize image to: 340x77

Traceback (most recent call last):

  File "D:\Program Files\python-3.7.9\lib\site-packages\PIL\JpegImagePlugin.py", line 630, in _save

    rawmode = RAWMODE[im.mode]

KeyError: 'RGBA'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):

  File "D:/py/test-three/test_01.py", line 13, in <module>

    im.save('thumbnail.jpg', 'jpeg')

  File "D:\Program Files\python-3.7.9\lib\site-packages\PIL\Image.py", line 2212, in save

    save_handler(self, fp, filename)

  File "D:\Program Files\python-3.7.9\lib\site-packages\PIL\JpegImagePlugin.py", line 632, in _save

    raise OSError(f"cannot write mode {im.mode} as JPEG") from e

OSError: cannot write mode RGBA as JPEG

进程已结束,退出代码1

查看报错信息以及save源代码初步断定与保存格式有关，保存png无异常，查找问题得知：png格式图片颜色为四通道RGBA，jpeg格式为三通道RGB，如要保存JPEG格式，需要转换成三通道RGB格式，即im = im.convert('RGB')。

另示例代码为im.save('thumbnail.jpg'，‘jpeg’)，Windows运行下im.save('thumbnail.jpeg')即可