Add Attachment ID as class on images

general.php

<?php

/**
 * Attachment ID on Images
 *
 * @author Bill Erickson
 * @link http://www.billerickson.net/code/add-attachment-id-class-images/
 */
function be_attachment_id_on_images( $attr, $attachment ) {
	if( !strpos( $attr['class'], 'wp-image-' . $attachment->ID ) )
		$attr['class'] .= ' wp-image-' . $attachment->ID;
	return $attr;
}
add_filter( 'wp_get_attachment_image_attributes', 'be_attachment_id_on_images', 10, 2 );