Hassan - first, please rename the sum variable since there is a built-in MATLAB function of the same name. If you want to add all of these matrices together (assuming that all are 25x256) then just do
compositeImage = im{1};
for k=2:length(im)
compositeImage = compositeImage + im{k};
end
In your code, you are getting an error because you should be referencing im instead of sum when adding the kth image...since it is im that is the cell array.
