The design matrix, X, must be a matrix of real numbers. Each column of X corresponds to a single covariate. Each row of X corresponds to a single trial (one image will be generated per trial). So row i, column j controls how active covariate j is during trial i. For example
X = eye(5);
corresponds to 5 trials, with one covariate active per trial.
X = rand(10,5);
gives you 10 trials, with 5 covariates randomly active during each trial. Finally,
X = [eye(5) eye(5)]
gives you 5 trials, with 2 covariates active during each trial.