Update of Cesar Canassa's answer.
This will create a thumbnail of image.jpg
as image_thumb.jpg
:
from PIL import Image, ImageOps
fname = 'image.jpg'
size = (48,48)
thumb = ImageOps.fit(Image.open(fname), size, Image.ANTIALIAS)
thumb.save('{}_thumb.jpg'.format(fname[:fname.rfind('.')]), "JPEG")