Thumbnail getter for wordpress templates

// Logik Thumbnail Getter
function logik_thumbnail() {
    if (is_home()) { $img_url = get_option('logik_fbimgurl'); }
    elseif(has_post_thumbnail( $post->ID )) {
        $thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'thumbnail' );
        $img_url = $thumb['0'];
    } else {
    //function to call first uploaded image in functions file
    $files = get_children('post_parent='.get_the_ID().'&post_type=attachment
    &post_mime_type=image&order=desc');
      if($files) :
        $keys = array_reverse(array_keys($files));
        $j=0;
        $num = $keys[$j];
        $img_url=wp_get_attachment_thumb_url($num);
      endif;
    }
    if ( $img_url == '' ) {
        $img_url = get_option('logik_fbimgurl');
    }
    if ( $img_url == '' ) {
        get_option('logik_fbimgurl');
        $template=get_theme_root_uri();
        $img_url = $template .'/ThemeName/images/defaultimage.jpg';
    }
    echo $img_url;
}