You can use imcrop like this:
[rows, columns, numberOfColorChannels] = size(rgbImage);
n = 40;
croppedImage = imcrop(rgbImage, [n, n, columns - 2*n, rows - 2 * n);
The above code works for either an RGB full color image, or a gray scale image.
Or you can use indexing. For a full color image, you can do
croppedImage = rgbImage(n:rows-2*n, n:columns-2*n, :); % For RGB image
Or, for a gray scale image:
croppedImage = grayScaleImage(n:rows-2*n, n:columns-2*n); % For a gray scale image
