how to subtract a derivative image from original image?

let us assume an image I of 512*512 and i want to subtract derivative of this image that is image I' of 512*511. so how to do , please help. code: clc; clear all; m=512; n=512; %peaks [x,y,z]=peaks; z=imresize(z,[m,n]); figure,imshow(z,[]); z1=diff(z,1,2); sub=????