first, i'm a real newbie so excuse me if i suck!

i got a wp theme that can display a category page with thumbs (of posts

    $cat_args = array(
      'orderby' => 'name',
      'order' => 'ASC',
   // 'number' => $num,
      'child_of' => 0,
      'show_count' => 1
      );
 $categories =   get_categories($cat_args); 
 foreach($categories as $category) {
echo '<a href="' . get_category_link( $category->term_id ) . '" title="' . sprintf( __( "Toutes les vidéos de %s" ), $category->name ) . '" ' . '><div class="video video_">';
echo '<h3>' . $category->name.'</h3>';


 $post_args = array(
  'numberposts' => 1,
  'category' => $category->term_id 
);
$posts = get_posts($post_args);
foreach($posts as $post) {
?>
    <div class="thumbnail">
        <?php if ( has_post_thumbnail() ) { echo the_post_thumbnail();}else{ ?>
            <img src="<?php echo get_post_meta($post->ID,   get_option('wptuber_thumbs'), true); ?>" alt="<?php the_title(); ?>" />
        <?php } ?>
        </div>
    <?php 
    } 
echo '<em>Toutes les videos ('.$category->count.')</em>';
echo '</div></a>';
} 
?>

but when i try to change to get the same wit "tag" every thing work fine but the thumb is not showing. can someone help me?

    $tag_args = array(
    'orderby' => 'name',
    'order' => 'ASC',
 // 'number' => $num,
    'child_of' => 0,
    'show_count' => 1
     );
$tags =   get_tags($tag_args); 
foreach($tags as $tag) {
   echo '<a href="' . get_tag_link( $tag->term_id ) . '" title="' . sprintf( __( "Toutes les videos de %s" ), $tag->name ) . '" ' . '><div class="video video_">';
echo '<h3>' . $tag->name.'</h3>';


$post_args = array(
  'numberposts' => 1,
  'tag' => $tag->term_id 
);
$posts = get_posts($post_args);
foreach($posts as $post) {
?>
    <div class="thumbnail">
        <?php if ( has_post_thumbnail() ) { echo the_post_thumbnail();}else{ ?>
            <img src="<?php echo get_post_thumbnail($post->ID, get_option('wptuber_thumbs'), true); ?>" alt="<?php the_title(); ?>" />
        <?php } ?>
        </div>
    <?php 
    }

    echo '<em>Toutes les videos de ('.$tag->count.')</em>';
echo '</div></a>';
} 

What am i missing?

Thanxs in advance