Вы используете изображение «RGB», которому не нужна палитра, поэтому getpalette () возвращает None.
Подробнее: https://pillow.readthedocs.io/en/4.1.x/reference/Image.html
Об индексированных изображениях (с палитрой): http://www.drububu.com/tutorial/image-types.html
Пример кода:
from PIL import Image
im = Image.open('image.png')
print(im.getpalette())
im2 = im.convert("P", palette=Image.ADAPTIVE, colors=256)
print(im2.getpalette())
im2.save('test.png')
Выход:
None
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