ic_get_attachment_id.php

/**
* Obtiene el id del attachment a partir del id de la categoría
* @param string taxonomy name 
* @param int term id
* @return int attachment id
*/
function ic_get_attachment_id( $taxonomy = 'category', $term_id ) {

	$args = array(
		'post_type'   => 'attachment',
		'numberposts' => -1,
		'post_status' => null,
		'tax_query'   => array(
			array(
				'taxonomy' => $taxonomy,
				'field'    => 'id',
				'terms'    => array( $term_id )
			)
		)
	);

	$attachments = get_posts( $args );

	foreach ( $attachments as $attachment ) {

		$id = $attachment->ID;

	}

	return  $id;

}