Is your field set to return the URL? or the object?
// ACF < 5.5.0
<img src="<?php the_field('image_field_name', $custom_term->taxonomy.'_'.$ $custom_term->term_id); ?>" alt="" class="img-responsive">
// ACF >= 5.5.0
<img src="<?php the_field('image_field_name', 'term_'.$custom_term->term_id); ?>" alt="" class="img-responsive">
See https://www.advancedcustomfields.com/resources/get-values-from-a-taxonomy-term/
If you are returning a image object/array instead of the url then see https://www.advancedcustomfields.com/resources/image/ for information on showing image fields.
