You can use conv2() or imfilter() to slide a 32 by 32 window across the image by one pixel at a time and get the mean. Nearly always an odd size (31 or 33) is used because then there are the same number of pixels to the left and right - the window is centered over the pixel. With an even number, the output and input images are shifted by a half pixel. Here's the code:
kernel = ones(32)/32^2; % Create averaging window.
output = conv2(grayImage, kernel, 'same'); % Get means.
If you want to move over in jumps of 32 pixels instead of 1 pixel, see my demos for blockproc (attached).
