<?php
                $args = array(
                  'orderby' => 'name',
                  'order' => 'ASC'
                );
            $categories = get_categories($args);
                foreach($categories as $category) {
                    echo '<a class="cover margin-ultimos" href="' . get_category_link( $category->term_id ) . '"><div id="cover-home" class="gray-shadow"><img src="'.(( $terms = get_the_category() ))? the_field( 'portada', 'category_'.$terms[0]->term_id ):'" alt="Portada'. get_the_title ( $ID ) . '" /></div></a>';
                }
            ?>