Never mind I found the solution. This is the code I used
<?php $category_id = get_queried_object_id(); ?>
<?php $cat_id = 'category_'.$category_id; ?>
<?php $image = get_field( 'cover', $cat_id); ?>
<img src="<?php echo $image['url']; ?>" alt="<?php echo $image['alt']; ?>" />
