Добавить свои варианты в srcset

Нашел

/**
* filter function to force wordpress to add our custom srcset values
* @param array  $sources {
*     One or more arrays of source data to include in the 'srcset'.
*
*     @type type array $width {
*          @type type string $url        The URL of an image source.
*          @type type string $descriptor The descriptor type used in the image candidate string,
*                                        either 'w' or 'x'.
*          @type type int    $value      The source width, if paired with a 'w' descriptor or a
*                                        pixel density value if paired with an 'x' descriptor.
*     }
* }
* @param array  $size_array    Array of width and height values in pixels (in that order).
* @param string $image_src     The 'src' of the image.
* @param array  $image_meta    The image meta data as returned by 'wp_get_attachment_metadata()'.
* @param int    $attachment_id Image attachment ID.

* @author: Aakash Dodiya
* @website: http://www.developersq.com
*/
add_filter( 'wp_calculate_image_srcset', 'dq_add_custom_image_srcset', 10, 5 );
function dq_add_custom_image_srcset( $sources, $size_array, $image_src, $image_meta, $attachment_id ){

	$image_basename = wp_basename( $image_meta['file'] );
	$image_baseurl = _wp_upload_dir_baseurl();
	// Uploads are (or have been) in year/month sub-directories.
	if ( $image_basename !== $image_meta['file'] ) {
		$dirname = dirname( $image_meta['file'] );

		if ( $dirname !== '.' ) {
			$image_baseurl = trailingslashit( $image_baseurl ) . $dirname;
		}
	}
        // get image baseurl
	$image_baseurl = trailingslashit( $image_baseurl );
	// check whether our custom image size exists in image meta
	if( array_key_exists('wt375_253', $image_meta['sizes'] ) ){
		// add source value to create srcset
		$sources[ $image_meta['sizes']['wt375_253']['width'] ] = array(
				 'url'        => $image_baseurl .  $image_meta['sizes']['wt375_253']['file'],
				 'descriptor' => 'w',
				 'value'      => $image_meta['sizes']['wt375_253']['width'],
		);
	}

        //return sources with new srcset value
	return $sources;
}