A quick look tells me that Alpaca probably contains 55 grayscale images whereas img is a color image with 3 channels corresponding to RGB. So if you want to stack color images, you need a 4-D array:
newArray = zeros(size(img,1), size(img,2), size(img,3), nbIm, 'uint8')
where nbIm if the number of images. Then:
newArray(:,:,:,idx) = img
for idx from 1 to nbIm.