Help with Multisite Thumbnail URLs

Hello,

I purchased a theme from themeforest.net that wasn’t tested on multisite and it’s having some trouble with thumbnail generation. I’ve narrowed it down to a problem with a script it’s using called thumbnail.php. I’m not good with php and am really hoping someone will know a few lines of code to help me fix it.

Here is the page that isn’t generating thumbnails correctly: http://www.kellyagarrett.com/category/blog/ (scroll down a few posts…also note, one on that page DOES work, so I don’t want to mess up that one!)

Anyway, here is the thumbnail script:

/*
 * Generate a thumbnail from an image url and return it's url
 *
 * @param string $img_url absolute url
 * @param int $width
 * @param int $height (optional)
 * @param boolean $cut (optional)
 */
function generate_thumb($img_url, $width, $height, $cut = true){

    $default_upload_dir = "wp-content";
    $upload_dir = get_option("upload_path");

    // cut the url
    if (strpos($img_url, $default_upload_dir))
    {
        $img = substr($img_url, strpos($img_url, $default_upload_dir));
    }
    else
    {
        $img = substr($img_url, strpos($img_url, $upload_dir));
    }
    // resize the image
    $thumb = image_resize($img,$width,$height,$cut);

    if (is_string($thumb))
    {
        $thumb_ = site_url() . '/' .  $thumb;
    }
    else
    {

        $thumb_ = site_url() . '/' . $img;
    }

    return $thumb_;
}

?>

The thumbnails are being returned as: http://www.kellyagarrett.com/http://www.kellyagarrett.com/files/etc… so, basically, it’s just duplicating the domain name and is finding the correct folder. The image that IS working currently, is located in a different uploads folder…I can’t figure out how to change the uploads folder so it doesn’t use a new folder every month.

Is anyone able to help me? It would be greatly appreciated!