[Plugin: NextGEN Gallery] access singlepic image’s URL

I’m using this code in my theme to retrieve and image from NextGen
<?php echo do_shortcode("[singlepic id= $custom_field_featuredImage w=780 h= float=]"); ?> where $custom_field_featuredImage is a number set with a custom field. It works fine, but I’d like to link to the full size image on click, like it would if I just used the shortcode in wp-admin. How can I access that data?

https://www.ads-software.com/extend/plugins/nextgen-gallery/