The following code lets me set a default background image. Is it possible to set multiple custom backgrounds?
add_theme_support( 'custom-background', array( 'default-image' => get_stylesheet_directory_uri() . '/images/image1.png', ) );
There is no direct way of doing that as far as I can see. But I got hold of this plugin which allows you to create Image Sets and set random background images -> Background Manager