modify image and save with exif data

[jwe.van.dijk at gmail.com]

I want to resize an image but retain the exif data
I now have:
import Image

img = Image.open('photo.jpg')
img.thumbnail((800, 800), Image.ANTIALIAS)
img.save('photo800.jpg', 'JPEG')

The saved image photo800.jpg has no exif info anymore.
I would so much like to have it retained in particular the exposure and gps data.

I use Python 2.7 with PIL 1.17 on Ubuntu 12.04
For exif reading/writing I use pyexiv2 0.3.2 but apparently you cannot add tags to an image that has none; only modifying existing ones seems to work.

Thanks for any helpful suggestions,
Janwillem