It supposes that you have a pre-loaded MNIST matrix and creates a mosaic of random samples.

mnist_mosaic.m

% Check this repository to create your own MNIST Matlab's matrix
% https://github.com/daniel-e/mnist_octave/

% Loading MNIST matrix file
load('mnist.mat');

% Creating an empty array of 100 samples
A = zeros(28, 28, 1, 100, 'uint8');

% Looping through 100 random samples
for i = 1:100
    % Gathering sample
    I = trainX(i,:);
    
    % Reshaping sample
    I = reshape(I, [28, 28]).';
    
    % Applying sample to array
    A(:,:,1,i) = I;
end

% Plotting montage
montage(A, 'size', [10 10]);

% Writing output
print('mnist.eps','-deps2', '-r300');