pop out

Meta key is added either in admin

add_action( 'created_product_cat', 'save_custom_taxonomy_image', 10, 2 );
function save_custom_taxonomy_image ( $term_id, $tt_id ) {
    if( isset( $_POST['brand_image'] ) && '' !== $_POST['brand_image'] ){
        $image = $_POST['brand_image'];
        add_term_meta( $term_id, 'brand_image', $image, true );
    }
}

Or by REST API

add_action('rest_api_init', 'register_rest_field_for_custom_taxonomy_location');
function register_rest_field_for_custom_taxonomy_location() {
    register_rest_field( 'product_cat',
        'brand_image',
        array(
            'get_callback'    => 'brand_image_get_term_meta_field',
            'update_callback' => 'brand_image_update_term_meta_field',
            'schema' => null));
}

//WRITE
function brand_image_update_term_meta_field( $value, $object, $field_name ) {
    if ( ! $value ) {
        return;
    }
    $id = (int) $object->term_id;
    $upload = wc_rest_upload_image_from_url( esc_url_raw( $value['src'] ) );
    if ( is_wp_error( $upload ) ) {
        return $upload;
    }
    $image_id = wc_rest_set_uploaded_image_as_attachment( $upload );
    return update_term_meta( $id, 'brand_image', $image_id );
}

//READ
function brand_image_get_term_meta_field( $object, $field_name, $request ) {
    $meta = get_term_meta( $object[ 'id' ], $field_name, true);
    $results = array( 'id' => $meta,
        'src' => wp_get_attachment_url($meta)
    );
    return $results;
}