For something as simple as this, PIL is not really needed - you can do it with numpy fliplr
.
import matplotlib.pyplot as plt
import numpy as np
im = np.flipud(plt.imread('so.jpg'))
plt.subplot(2, 1, 1)
plt.imshow(im)
plt.subplot(2, 1, 2)
plt.imshow(np.fliplr(im))
plt.show()
wolf revok cats !