code: public static function get_thumbnail_file( $meta, $size = 'medium' ) {
  
  		if ( ! isset( $meta[ 'sizes' ][ $size ] ) ) {
  			$file = FALSE;
  		} else {
  			$dir = wp_upload_dir();
  			$file_parts = array(
  				$dir[ 'basedir' ],
  				dirname( $meta[ 'file' ] ),
  				$meta[ 'sizes' ][ $size ][ 'file' ]
  			);
  
  			$file = implode( DIRECTORY_SEPARATOR, $file_parts );
  		}
  
  		return apply_filters( 'iac_attach_media_thumbnail_file', $file, $meta, $size );
  	}
docs: function to get the filesystem path to an image
  
  @param string $size
  @param array $meta (Return value of wp_get_attachment_metadata() )
  @return string|false