If you ever need to use Advanced Custom Fields to load an image or attachment on a category, here’s the template code you need. 🙂

// vars
$queried_object = get_queried_object(); 
$taxonomy = $queried_object->taxonomy;
$term_id = $queried_object->term_id;  

// load thumbnail for this taxonomy term (term object)
$thumbnail = get_field('thumbnail', $queried_object);

// load thumbnail for this taxonomy term (term string)
$thumbnail = get_field('thumbnail', $taxonomy . '_' . $term_id);

the_field( $thumbnail );