Store image into array for classification training (SOM)

Hi there, i currently have over 7000 images that i want to train for classifcation of certain characters. I tried loading them into a matrix but it ends up as a 3D array like this:

arrayImage 128x128x7112 double

clear,clc;
files = dir('*.png');
num = length(files);
arrayImage = zeros(128,128,num);
i = 1;

for file = files'
    img = imread(file.name);
    arrayImage(:,:,i) = img;
    i = i + 1;
end

I think matlab's neural net clustering only reads in a 2D matrix. How can i store them as a 2D array so i can load them into SOM for training. I attached some examples of images i want to store and use for training.