import numpy as np
import cv2

img = cv2.imread("pxpuz.jpg", cv2.IMREAD_COLOR)
white_pixels = np.where(np.all(img == (255, 255, 255), axis=-1))
# Make list of tuples (X, Y)
white_coords = zip(white_pixels[1], white_pixels[0])
# Sort in ascending order, by (Y, X)
sorted_coords = sorted(white_coords, key=lambda v: (v[1], v[0]))
if sorted_coords:
    # First coordinate pair is the top-left most white pixel...
    cv2.circle(img, sorted_coords[0], 5, (255, 0, 0), -1)
cv2.imwrite("pxpuz_result.png", img)