Passing a single variable - the easiest way?

I have a client that wants a slideshow. XML. And I am using the simple thumbnails slideshow tutorial on here.

However, they want the thumbnails, when clicked, to open to a new window that displays the full sized picture in a customizable. (So peple can have multiple windows open.)

I have been reading for hours about passing variables, looking into loadVars, flashVars, query strings, etc etc etc. And I am just beat.

I can’t find which is BEST to use. I just need to be able to pass WHICH thumbnail they clicked on to the popup so that it will load.

Can someone point me in the right direction?