from PIL import Image

i1 = Image.open('./image_1.png')

for i in range(18):
    i2 = Image.open('./image_%s.png'% i)
    pairs = zip(i1.getdata(), i2.getdata())
    if len(i1.getbands()) == 1:
        # for gray-scale jpegs
        dif = sum(abs(p1-p2) for p1,p2 in pairs)
    else:
        dif = sum(abs(c1-c2) for p1,p2 in pairs for c1,c2 in zip(p1,p2))
    ncomponents = i1.size[0] * i1.size[1] * 3
    print ("Difference (percentage):", (dif / 255.0 * 100) / ncomponents)