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This is how you might get the colour value for each block in Python, using pillow, the standard image processing library. You could do the posterisation as well.

Code formatting doesn't seem to quite work, // is integer division, not a comment

from PIL import Image

BLOCK_SIZE = 40
img = Image.open('image.png')
img_x, img_y = img.size

for x_block in range(img_x // BLOCK_SIZE):
    for y_block in range(img_y // BLOCK_SIZE):
        colour = img.getpixel((x_block * BLOCK_SIZE, y_block * BLOCK_SIZE))
        print('x:{}, y:{}; rgb:{}'.format(x_block, y_block, colour))

This would give you output like:

x:0, y:0; rgb:(65, 12, 11)
x:0, y:1; rgb:(197, 56, 120)
x:0, y:2; rgb:(245, 18, 90)

where x, y coordinates refer to blocks, not pixels.