WordPress MU

I am trying to get timthumb working with wpmu. Having followed the directions on his site, I am able to successfully grab a 'thumb' custom field and have it show up in my post using the following code:

function get_image_path ($post_id = null) {
	if ($post_id == null) {
		global $post;
		$post_id = $post->ID;
	}
	$theImageSrc = get_post_meta($post_id, 'thumb', 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;
}

What I would like to do is have a function that will search the post for the first image, and then return that image url 'properly formatted for wpmu with the 'blogs.dir/#/files' within the url so that timthumb can function.

Here is the function that I am using currently which works with non-mu sites, but does not work with wpmu.

/*this function allows users to use the first image in their post as their thumbnail*/
function catch_that_image() {
	global $post, $posts;
	$first_img = '';
	ob_start();
	ob_end_clean();
	$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
	$first_img = $matches [1] [0];

	return $first_img;
}

Obviously that function does not return the wpmu image url that is needed, as the blogs.dir/blogid# is missing from the url.

Could someone please help me with this?