How to put a random order on the slider?

Hello everybody, I am looking for a new theme for my site, and I like Full Frame. I started to test it locally, with a child theme. I’ve got a little problem with the slider. I would like to get a random display of the featured images from the pages. Even with the 'fullframe_page_slider' function in the child-theme to modify it, with 'order' => 'rand': it does not work. When I refresh the page, the images appear even in the same order. How can I do ?