Considering im is a gray level image, and the central part of the image considered as mid section of the image as per its rows and clumns number or you may change the segment as per your requiremnt.
original_im=im;
[r,c]=size(im);
cent_part=im(round(r/4):round(3*r/4),round(c/4):round(3*r/4));
im(round(r/4):round(3*r/4),round(c/4):round(3*r/4))=cent_part+imnoise(cent_part,'gaussian');
% You can add any other types of noises here,'gaussian' just for example
imshow([original_im,im]);
title('Priginal Image Noise Added Image');
