Theme
functions.php
– add to display function
– add to save function

Plugins

simple-lightbox
templates/default/layout.html
– add output variable
js/lib.js
– add placeholder value (line 53)
– push to items array (345)
– add function to retrieve and set value (807)
– set value in html (945)

cleaner-gallery
gallery.php
– add to page output (line 121)
cleaner-gallery.php
– add as attribute to image tag (135)