I ended up using pyexiv2 to modify the image's metadata, like this:
>>> md5sum('photo.jpg')
'89dd603a0ce14750799a5144a56fbc12'
>>> image = pyexiv2.ImageMetadata('photo.jpg')
>>> image.read()
>>> image['Exif.Image.ImageDescription'] = '%030x' % random.randrange(256**15)
>>> image.write()
>>> md5sum('photo.jpg')
'426cc91835e7f4f5e92c5a48850adc05'