Lightbox – add gallery

Hello there,

I recently started to update a clients wordpress site. I wanted to upload some new pics for the media section, but all I found was this

[su_custom_gallery source=”media: 502,503,499,501,500,441,483,482″ link=”lightbox” width=”300″ height=”300″ title=”always”]

And there was no Lightplugin installed. My question: how do I add another pic?

I presume I should add the number of the uploaded pic, but where do I find it,
thanx in advance