Add classes to images on single entries in Sell Media

gistfile1.php

<?php

/**
 * Add more classes to main images appearing on single entries in Sell Media.
 * Change 'lightbox' below to whatever classes you want to add.
 */
 function  sell_media_filter_single_image_class( $classes ) {
  
     if ( is_singular( 'sell_media_item' ) ) {
     	$classes[] = 'lightbox';
     }
     
     return $classes;
     
 }
 add_filter( 'sell_media_image_class', 'sell_media_filter_single_image_class' );