Python error : index out of bounds

Because your image doesn't have squarred shape, reshape it before you go through your loop

EDIT

We can iterate through each row/column position and save to a file as below.It will take very long time depending upon file size.

Instead of writing your own function, you may want to take advantage of inbuilt binary save (which is more efficient) as

np.save('matrix.py', np_array)

You can load this file as np array and manipulate

Or as a text file using np.save [ will take longer ]

np.save('matrix.txt', np_array)

Working Code:

import imageio
import numpy as np
im = imageio.imread("9v9zU.png")
matrice_pixels=open("matric.txt","wb")
nx,ny = im.shape
for i in range(nx):
    line=""
    for j in range(ny):
        line+=repr(im[i][j])
    matrice_pixels.write(line+'\n')

matrice_pixels.close()

#Save as Binary data
np.save('matrix1.npy', im)

#Save as Human readable data
np.savetxt('matrix1.txt', im)

Alternately, you may want to look into off the shelf libraries that will do what you are intending to do. For e.g. This SO link discusses how to remove section of the picture based upon its color using PIL library.

Also , in future, please DO NOT post a picture of your code. Copy/pase to SO window so that we can copy and modify. In this case I had write everything down line by line to test(thankfully code was not that long).