import cv2
import numpy as np
img = cv2.imread("your_image.jpg")
x,y=img.shape[:2]
ma=max(x,y)
blank = np.zeros((ma,ma,3), np.uint8)
if y>x:
    blank[(y-x)/2:(y-x)/2+x,0:y]=img
if x>y:
    blank[0:x,(x-y)/2:(x-y)/2+y]=img
cv2.imshow('result', blank)
cv2.imwrite("result.jpg",blank)
cv2.waitKey(0)