I cannot check the source code at the moment, but it looks like the right place.
Pay attention to the if clause: it may be that your modification is skipped entirely.

If you want to [b]always[/b] add the parameter, I'd advise you modify the first line of code:
path = webPath+"/" + adminDir + "/i.php?t=true&a="+albumUrl+"&i="+imageUrl;

Also: once ZenphotoPress has generated a URL, it won't be changed automatically even if you modify the code.