>>> image.shape
(800, 600, 3)
>>> chex.shape
(800, 600, 3)
>>> mask.shape
(800, 600)

image[mask == 0,...] = chex

idx=(mask==0)
image[idx]=chex[idx]

image[mask[:] == 0,...] = chex[mask[:] == 0,...]

# first create mini-versions of your arrays:
mask = NP.random.random_integers(0, 1, 48).reshape(8, 6)
img = NP.random.random_integers(3, 9, 8*6*3).reshape(8, 6, 3)
chk = NP.ones((8, 6, 3))

# all the work done in these two lines
mask = mask[:,:,NP.newaxis]
res = NP.where(mask==0, chk, img)

import matplotlib.pyplot as plt
from scipy.ndimage import rotate


cat = plt.imread('cat.jpeg')
bg = plt.imread('background.jpeg')


rotcat = rotate(cat, angle=8, reshape=True) ## rotating creates some black edges
height, width, _ = rotcat.shape

bgcopy = bg.copy() ## create a copy of the background; paste on copy

x, y = 40, 50
bgcopy[x:x+height, y:y+width] = rotcat
plt.imsave('cat-on-bg-mask.jpg', bgcopy)

mask_ind = (bgcopy == 0)
bgcopy[mask_ind] = bg[mask_ind]
plt.imsave('cat-on-bg.jpg', bgcopy)