Nearly there. You can't use randperm on a cell array, but you can use it to get a random permutation of indices that you then use to reorder the cell array:
sequence = {image1 image2};
randidx = randperm(numel(sequence));
randsequence = sequence(randidx);
To display the first image in randsequence:
imshow(randsequence{1}); %note the curly brackets and not round brackets
