How to get featured image from specific post not from the latest updated post?

I'm using Network theme for the main site, it's very nice for it's display way of the frontpage, but...

Not all the posts have it's own featured thumbnail, then it will display a default thumbnail image. That would make index looks weird.

How to get featured image from specific post for example "$post->ID=2" for every site not from the latest updated post for the frontpage?

I think it is the code from custom-functions.php in line 613 that effects the thumbnail display , but I don't know how to customize it to get the thumbnail from the post with it's id=2.
<img src="<?php bloginfo('template_directory'); ?>/library/functions/timthumb.php?src=<?php echo $thumbnail; ?>&h=108&w=227&zc=1&multisite=true&blogdirid=<?php echo ( preg_match('/^/?blogs.dir/', $upload_dir) ) ? $blog : 0; ?>" alt="" style="width:227px; height: 108px;" />

Any assistance would be appreciated.