Excerpt of WPENGINE's mu-plugins/wpengine-common/plugin.php file

plugin.php

	/**
	 * This handles CDN replacement for srcset images.
	 *
	 * @since 2.2.10
	 * @filter wp_calculate_image_srcset
	 * @param array $sources The image metadata as returned by 'wp_get_attachment_metadata()'.
	 */
	public function wpe_cdn_srcset( $sources ) {
		if ( defined( 'WPE_SRCSET_CDN_DISABLED' ) && true === WPE_SRCSET_CDN_DISABLED ) {
			return $sources;
		}

		foreach ( $sources as $source ) {
			// The actual CDN replacement.
			if ( $new_path = $this->wpe_cdn_replace( $sources[ $source['value'] ]['url'] ) ) {
				$sources[ $source['value'] ]['url'] = $new_path;
			}
		}

		return $sources;
	}