To randomize media, you can add them into groups using the Group element.
Groups allow you to randomize elements and create more complex experiments. This is similar to a folder where you put items and set the order in which they are displayed.
It is possible to add any element to a group, be it instructions, media, shelf, question or another group. For groups within groups, two levels of nesting are supported. You can choose to randomize the order and number of group elements shown.
To learn more about each element type, check the article about designing experiments in Sticky.