Help adding thumbnails to theme using WPMU

[PowerMedia]

We are trying to add thumbnails to story posts on our homepage using timthumb, but are running into some difficulties. We found an article giving instructions, but can't figure out where to place the code. they suggest reaching out to the Theme creator to figure out where to place code. Here is the code they tell us to add:

function get_image_path ($post_id = null) {
if ($post_id == null) {
global $post;
$post_id = $post->ID;
}
$theImageSrc = get_post_meta($post_id, 'Image', true);
global $blog_id;
if (isset($blog_id) && $blog_id > 0) {
$imageParts = explode('/files/', $theImageSrc);
if (isset($imageParts[1])) {
$theImageSrc = '/blogs.dir/' . $blog_id . '/files/' . $imageParts[1];
}
}
return $theImageSrc;
}

Where in the Streamline theme should we put this code? Thanks for any help you can provide.