Random Order for Slider

Hi - I want the images in the slider to be in a random order each time there is a visitor to the page, rather than in id order. The slider is a category slider on the homepage, calling post fomr one category only. I've had a look in catcheverest-functions.php and changed 'orderby- => 'post__in' to 'orderby' => 'rand' in function catcheverest_category_sliders() but it hasn't had any effect! Any ideas?