If you'd used the debugger to see what was happening with your loops, you would have quickly realised that you have one loop too many, the outer loop. Your i should be a counter not a loop, that is incremented by one inside the inner most loop:
i = 1;
for ii = 1:64
for iii = 1:8
img{i} = cell_image{ii}(:, :, iii); %do not use image as a variable name. It's a matlab function
i = i + 1;
end
end
However, even better is to learn to manipulate cell arrays, the above is achieved more simply with:
img = cat(3, cell_image{:});
img = squeeze(num2cell(img, [1 2])); %I would actually not bother with this step and simply keep the 3d matrix
