Its Pretty Simple. You have not made 'b' into an array. do it and its all done.
i.e. Change it to the following
a=imread('C:\Users\a.png'); disp(a); for i=1:256 for j=1:256 b(i,j,:) = de2bi(a(i,j),8); disp(b) end end
Please let me know if in case of further doubts.