<?php
add_filter('wp_get_attachment_image_attributes', 'interchange_the_images', 10, 2);

function interchange_the_images ( $attr, $attachment )
{
	/* Use ID to get the attachment object */
	$lg_hero_array = wp_get_attachment_image_src( $attachment->ID, 'large', true ); //Large Hero
	$md_hero_array = wp_get_attachment_image_src( $attachment->ID, 'medium', true ); // Medium Hero
	$sm_hero_array = wp_get_attachment_image_src( $attachment->ID, 'thumbnail', true ); // Mobile Hero

	/* Grab the url from the attachment object */
	$hero_lg = $lg_hero_array[0]; //Large Hero
	$hero_md = $md_hero_array[0]; // Medium Hero
	$hero_sm = $sm_hero_array[0]; // Mobile Hero

	$attr['interchange'] = '[' . $hero_lg . ', (default)],[' . $hero_sm . ', (small)],[' . $hero_md . ', (medium)],[' . $hero_lg . ', (large)]';
	return $attr;
}