Yes, any custom PHP code you want to add would go in your Child theme’s functions.php file. If you get a white screen afterwards, it means your code isn’t correct and you’re getting a fatal error because of it.
to 2. lightbox2 is not integrated in the plugin. I need to download this and put it somewhere in the theme. I use the child theme. But I don’t know where I must insert the code. Here is a link scroll down , I must insert this code somewhere in the header?!
And in your functions.php is also where you’d include any scripts or CSS files. —
Keep in mind however that support does not include doing customizations: http://support.themeblvd.com/help/#terms