How can i use zero-padding in an image?

There is a built-in function for that. It's called padarray().

Demo

grayImage = imread('cameraman.tif');
whos grayImage % 256 x 256
windowSize = 3; % Filter window's full width.
% Compute the number of layers of pixels with value 0 to surround the image with.
numLayers = floor(windowSize/2) % Will be 1
grayImage2 = padarray(grayImage, [numLayers, numLayers]); % Create new image, or use img if you want to replace the image instead of creating a new one.
whos grayImage2 % Will be 258 x 258M