The field you are trying to show is on the attachments. Using get_field where you are using it is trying to get the field from the current post.
You need to query the attachments and get the post with the value. This will get a random attachment post.
$args = array(
'post_type' => 'attachment',
'orderby' => 'rand',
'posts+per_page' => 1,
'meta_query' => array {
array (
'key' => 'image_category',
'value' => 'banner_advert'
)
)
);
$my_query = new WP_Query($args);