How to obtain a group of pixels on an image

I have an image and I want to obtain a group of pixels with a distance of about 50 pixels each for both row and column for the entire image.

This is my code so far.

I = imread('balloon.jpg');
[row, col, channel] = size(I);
a = 0;
X = 1 : 50 : col;
for x = 1 : 50 : col
    for y = 1:50:row
    I(50,x,:) = [255,0,0];
    I(y, 50, :) = [255,0,0];
    end
end
imshow(I)    

Answer 1

[KSSV]

If I is an image, you can use:

iwant = I (1: 5: end, 1: 5: end, :);