The third parameter they mean the third parameter of the function.
the parameters are separated by comma. like this:
$site_session->url(param1, param2, param3)
So if you find the needed line, check how many parameters already supplyed. if only one, then after first parameter add:
, 0, $image_name
In some cases the second paramater already supplyed (something like , "&") , then all you need to add is:
, $image_name